R. S. Ball, Great Astronomers

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Great Astronomers

Great Astronomers

R. S. Bal l

Preface Introduction

PTOLEMY.

COPERNICUS.

TYCHO BRAHE.

GALILEO.

KEPLER.

ISAAC NEWTON.

FLAMSTEED. HALLEY.

BRADLEY.

WILLIAM HERSCHEL.

LAPLACE.

BRINKLEY.

JOHN HERSCHEL.

THE EARL OF ROSSE.

AIRY. HAMILTON.

LE VERRIER.

ADAMS.

his page copyright © 2000 Blackmask Online.

PREFACE.

has been my object in these pages to present the life of each astronomer in such detail aenable the reader to realise in some degree the man's character and surroundings; and I

ave endeavoured to indicate as clearly as circumstances would permit the main features oe discoveries by which he has become known.

here are many types of astronomers--from the stargazer who merely watches the heavensthe abstract mathematician who merely works at his desk; it has, consequently, been

ecessary in the case of some lives to adopt a very different treatment from that which

le:///D|/Great Astronomers (Ball, R. S.)/greatastronomers2.htm (1 of 156) [03.04.2007 19:19:46]



Great Astronomers

emed suitable for others.

hile the work was in progress, some of the sketches appeared in "Good Words." The chapn Brinkley has been chiefly derived from an article on the "History of Dunsink Observatoryhich was published on the occasion of the tercentenary celebration of the University of ublin in 1892, and the life of Sir William Rowan Hamilton is taken, with a few alterations amissions, from an article contributed to the "Quarterly Review" on Graves' life of the great

athematician. The remaining chapters now appear for the first time. For many of the factsontained in the sketch of the late Professor Adams, I am indebted to the obituary noticeritten by my friend Dr. J.W.L. Glaisher, for the Royal Astronomical Society; while with regathe late Sir George Airy, I have a similar acknowledgment to make to Professor H.H.

urner. To my friend Dr. Arthur A. Rambaut I owe my hearty thanks for his kindness in aidie in the revision of the work.

S.B. The Observatory, Cambridge. October, 1895

INTRODUCTION.

f all the natural sciences there is not one which offers such sublime objects to the attentiothe inquirer as does the science of astronomy. From the earliest ages the study of the st

as exercised the same fascination as it possesses at the present day. Among the mostimitive peoples, the movements of the sun, the moon, and the stars commanded attentioom their supposed influence on human affairs.

he practical utilities of astronomy were also obvious in primeval times. Maxims of extrementiquity show how the avocations of the husbandman are to be guided by the movementse heavenly bodies. The positions of the stars indicated the time to plough, and the time t

ow. To the mariner who was seeking a way across the trackless ocean, the heavenly bodiefered the only reliable marks by which his path could be guided. There was, accordingly, mulus both from intellectual curiosity and from practical necessity to follow the movementhe stars. Thus began a search for the causes of the ever-varying phenomena which the

eavens display.

any of the earliest discoveries are indeed prehistoric. The great diurnal movement of theeavens, and the annual revolution of the sun, seem to have been known in times far morencient than those to which any human monuments can be referred. The acuteness of thearly observers enabled them to single out the more important of the wanderers which weow call planets. They saw that the star-like objects, Jupiter, Saturn, and Mars, with the moonspicuous Venus, constituted a class of bodies wholly distinct from the fixed stars amonghich their movements lay, and to which they bear such a superficial resemblance. But theenetration of the early astronomers went even further, for they recognized that Mercury aelongs to the same group, though this particular object is seen so rarely. It would seem th




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clipses and other phenomena were observed at Babylon from a very remote period, while ost ancient records of celestial observations that we possess are to be found in the Chine

nnals.

he study of astronomy, in the sense in which we understand the word, may be said to havommenced under the reign of the Ptolemies at Alexandria. The most famous name in theience of this period is that of Hipparchus who lived and worked at Rhodes about the year

60BC. It was his splendid investigations that first wrought the observed facts into a cohereanch of knowledge. He recognized the primary obligation which lies on the student of the

eavens to compile as complete an inventory as possible of the objects which are there to bund. Hipparchus accordingly commenced by undertaking, on a small scale, a task exactlymilar to that on which modern astronomers, with all available appliances of meridian circlend photographic telescopes, are constantly engaged at the present day. He compiled atalogue of the principal fixed stars, which is of special value to astronomers, as being the

arliest work of its kind which has been handed down. He also studied the movements of thn and the moon, and framed theories to account for the incessant changes which he saw

ogress. He found a much more difficult problem in his attempt to interpret satisfactorily tomplicated movements of the planets. With the view of constructing a theory which shouldve some coherent account of the subject, he made many observations of the places of thandering stars. How great were the advances which Hipparchus accomplished may beppreciated if we reflect that, as a preliminary task to his more purely astronomical laboursad to invent that branch of mathematical science by which alone the problems he proposeould be solved. It was for this purpose that he devised the indispensable method of lculation which we now know so well as trigonometry. Without the aid rendered by this

eautiful art it would have been impossible for any really important advance in astronomica

lculation to have been effected.

ut the discovery which shows, beyond all others, that Hipparchus possessed one of theaster-minds of all time was the detection of that remarkable celestial movement known ae precession of the equinoxes. The inquiry which conducted to this discovery involved aost profound investigation, especially when it is remembered that in the days of Hipparche means of observation of the heavenly bodies were only of the rudest description, and th

vailable observations of earlier dates were extremely scanty. We can but look withtonishment on the genius of the man who, in spite of such difficulties, was able to detect

ch a phenomenon as the precession, and to exhibit its actual magnitude. I shall endeavoexplain the nature of this singular celestial movement, for it may be said to offer the first

stance in the history of science in which we find that combination of accurate observationth skilful interpretation, of which, in the subsequent development of astronomy, we have any splendid examples.

he word equinox implies the condition that the night is equal to the day. To a resident on quator the night is no doubt equal to the day at all times in the year, but to one who livesny other part of the earth, in either hemisphere, the night and the day are not generally




Great Astronomers

ny other discoverer whose authority on the subject of the movements of the heavenly bodas held sway over the minds of men for so long a period as the fourteen centuries duringhich his opinions reigned supreme. The doctrines he laid down in his famous book, "Themagest," prevailed throughout those ages. No substantial addition was made in all that tithe undoubted truths which this work contained. No important correction was made of thrious errors with which Ptolemy's theories were contaminated. The authority of Ptolemy aall things in the heavens, and as to a good many things on the earth (for the same

ustrious man was also a diligent geographer), was invariably final.

hough every child may now know more of the actual truths of the celestial motions than eolemy knew, yet the fact that his work exercised such an astonishing effect on the humantellect for some sixty generations, shows that it must have been an extraordinaryoduction. We must look into the career of this wonderful man to discover wherein lay thecret of that marvellous success which made him the unchallenged instructor of the humace for such a protracted period.

nfortunately, we know very little as to the personal history of Ptolemy. He was a native ofgypt, and though it has been sometimes conjectured that he belonged to the royal familiee same name, yet there is nothing to support such a belief. The name, Ptolemy, appears

ave been a common one in Egypt in those days. The time at which he lived is fixed by thect that his first recorded observation was made in 127 AD, and his last in 151 AD. When w

dd that he seems to have lived in or near Alexandria, or to use his own words, "on thearallel of Alexandria," we have said everything that can be said so far as his individuality isoncerned.

olemy is, without doubt, the greatest figure in ancient astronomy. He gathered up thesdom of the philosophers who had preceded him. He incorporated this with the results of

wn observations, and illumined it with his theories. His speculations, even when they were we now know, quite erroneous, had such an astonishing verisimilitude to the actual facts

ature that they commanded universal assent. Even in these modern days we notnfrequently find lovers of paradox who maintain that Ptolemy's doctrines not only seem trut actually are true.

the absence of any accurate knowledge of the science of mechanics, philosophers in ear

mes were forced to fall back on certain principles of more or less validity, which they derivom their imagination as to what the natural fitness of things ought to be. There was noeometrical figure so simple and so symmetrical as a circle, and as it was apparent that theeavenly bodies pursued tracks which were not straight lines, the conclusion obviouslyllowed that their movements ought to be circular. There was no argument in favour of thotion, other than the merely imaginary reflection that circular movement, and circularovement alone, was "perfect," whatever "perfect" may have meant. It was further believebe impossible that the heavenly bodies could have any other movements save those whi

ere perfect. Assuming this, it followed, in Ptolemy's opinion, and in that of those who cam




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ter him for fourteen centuries, that all the tracks of the heavenly bodies were in some waother to be reduced to circles.

olemy succeeded in devising a scheme by which the apparent changes that take place in eavens could, so far as he knew them, be explained by certain combinations of circularovement. This seemed to reconcile so completely the scheme of things celestial with theeometrical instincts which pointed to the circle as the type of perfect movement, that we c

ardly wonder Ptolemy's theory met with the astonishing success that attended it. We shallerefore, set forth with sufficient detail the various steps of this famous doctrine.

olemy commences with laying down the undoubted truth that the shape of the earth isobular. The proofs which he gives of this fundamental fact are quite satisfactory; they aredeed the same proofs as we give today. There is, first of all, the well-known circumstancehich our books on geography remind us, that when an object is viewed at a distance acroe sea, the lower part of the object appears cut off by the interposing curved mass of wate

he sagacity of Ptolemy enabled him to adduce another argument, which, though not quitebvious as that just mentioned, demonstrates the curvature of the earth in a very impressivanner to anyone who will take the trouble to understand it. Ptolemy mentions that travellho went to the south reported, that, as they did so, the appearance of the heavens at nignderwent a gradual change. Stars that they were familiar with in the northern skies gradunk lower in the heavens. The constellation of the Great Bear, which in our skies never set

uring its revolution round the pole, did set and rise when a sufficient southern latitude hadeen attained. On the other hand, constellations new to the inhabitants of northern climesere seen to rise above the southern horizon. These circumstances would be quite

compatible with the supposition that the earth was a flat surface. Had this been so, a littleflection will show that no such changes in the apparent movements of the stars would bee consequence of a voyage to the south. Ptolemy set forth with much insight thegnificance of this reasoning, and even now, with the resources of modern discoveries to hs, we can hardly improve upon his arguments.

olemy, like a true philosopher disclosing a new truth to the world, illustrated and enforceds subject by a variety of happy demonstrations. I must add one of them, not only on accoits striking nature, but also because it exemplifies Ptolemy's acuteness. If the earth were

at, said this ingenious reasoner, sunset must necessarily take place at the same instant, natter in what country the observer may happen to be placed. Ptolemy, however, proved te time of sunset did vary greatly as the observer's longitude was altered. To us, of courseis is quite obvious; everybody knows that the hour of sunset may have been reached inreat Britain while it is still noon on the western coast of America. Ptolemy had, however, fthose sources of knowledge which are now accessible. How was he to show that the sun

ctually did set earlier at Alexandria than it would in a city which lay a hundred miles to theest? There was no telegraph wire by which astronomers at the two Places could




Great Astronomers

ommunicate. There was no chronometer or watch which could be transported from place tace; there was not any other reliable contrivance for the keeping of time. Ptolemy'sgenuity, however, pointed out a thoroughly satisfactory method by which the times of suntwo places could be compared. He was acquainted with the fact, which must indeed hav

een known from the very earliest times, that the illumination of the moon is derived entireom the sun. He knew that an eclipse of the moon was due to the interposition of the earthich cuts off the light of the sun. It was, therefore, plain that an eclipse of the moon must

phenomenon which would begin at the same instant from whatever part of the earth theoon could be seen at the time. Ptolemy, therefore, brought together from various quartere local times at which different observers had recorded the beginning of a lunar eclipse. Hund that the observers to the west made the time earlier and earlier the further away theations were from Alexandria. On the other hand, the eastern observers set down the houter than that at which the phenomenon appeared at Alexandria. As these observers allcorded something which indeed appeared to them simultaneously, the only interpretationas, that the more easterly a place the later its time. Suppose there were a number of bservers along a parallel of latitude, and each noted the hour of sunset to be six o'clock,

en, since the eastern times are earlier than western times, 6 p.m. at one station A willorrespond to 5 p.m. at a station B sufficiently to the west. If, therefore, it is sunset to thebserver at A, the hour of sunset will not yet be reached for the observer at B. This provesonclusively that the time of sunset is not the same all over the earth. We have, however,ready seen that the apparent time of sunset would be the same from all stations if the eaere flat. When Ptolemy, therefore, demonstrated that the time of sunset was not the samarious places, he showed conclusively that the earth was not flat.

s the same arguments applied to all parts of the earth where Ptolemy had either been

mself, or from which he could gain the necessary information, it followed that the earth,stead of being the flat plain, girdled with an illimitable ocean, as was generally supposed,ust be in reality globular. This led at once to a startling consequence. It was obvious thatere could be no supports of any kind by which this globe was sustained; it thereforellowed that the mighty object must be simply poised in space. This is indeed an astonishinoctrine to anyone who relies on what merely seems the evidence of the senses, withoutving to that evidence its due intellectual interpretation. According to our ordinary experiene very idea of an object poised without support in space, appears preposterous. Would it ll? we are immediately asked. Yes, doubtless it could not remain poised in any way in whi

e try the experiment. We must, however, observe that there are no such ideas as upwarddownwards in relation to open space. To say that a body falls downwards, merely meansat it tries to fall as nearly as possible towards the centre of the earth. There is no onerection along which a body will tend to move in space, in preference to any other. This me illustrated by the fact that a stone let fall at New Zealand will, in its approach towards tharth's centre, be actually moving upwards as far as any locality in our hemisphere isoncerned. Why, then, argued Ptolemy, may not the earth remain poised in space, for as arections are equally upward or equally downward, there seems no reason why the earthould require any support? By this reasoning he arrives at the fundamental conclusion tha




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e earth is a globular body freely lying in space, and surrounded above, below, and on alldes by the glittering stars of heaven.

he perception of this sublime truth marks a notable epoch in the history of the gradualevelopment of the human intellect. No doubt, other philosophers, in groping after knowleday have set forth certain assertions that are more or less equivalent to this fundamentaluth. It is to Ptolemy we must give credit, however, not only for announcing this doctrine,

r demonstrating it by clear and logical argument. We cannot easily project our minds bace conception of an intellectual state in which this truth was unfamiliar. It may, however, bell imagined that, to one who thought the earth was a flat plain of indefinite extent, it woe nothing less than an intellectual convulsion for him to be forced to believe that he stoodpon a spherical earth, forming merely a particle relatively to the immense sphere of theeavens.

hat Ptolemy saw in the movements of the stars led him to the conclusion that they wereight points attached to the inside of a tremendous globe. The movements of this globe

hich carried the stars were only compatible with the supposition that the earth occupied itentre. The imperceptible effect produced by a change in the locality of the observer on thepparent brightness of the stars made it plain that the dimensions of the terrestrial globe me quite insignificant in comparison with those of the celestial sphere. The earth might, in fe regarded as a grain of sand while the stars lay upon a globe many yards in diameter.

o tremendous was the revolution in human knowledge implied by this discovery, that we cell imagine how Ptolemy, dazzled as it were by the fame which had so justly accrued to hiled to make one further step. Had he made that step, it would have emancipated the

uman intellect from the bondage of fourteen centuries of servitude to a wholly monstrousotion of this earth's importance in the scheme of the heavens. The obvious fact that the sue moon, and the stars rose day by day, moved across the sky in a glorious never-endingocession, and duly set when their appointed courses had been run, demanded some

xplanation. The circumstance that the fixed stars preserved their mutual distances from yeyear, and from age to age, appeared to Ptolemy to prove that the sphere which containeose stars, and on whose surface they were believed by him to be fixed, revolved completound the earth once every day. He would thus account for all the phenomena of rising antting consistently with the supposition that our globe was stationary. Probably this

pposition must have appeared monstrous, even to Ptolemy. He knew that the earth was gantic object, but, large as it may have been, he knew that it was only a particle in

omparison with the celestial sphere, yet he apparently believed, and certainly succeeded inersuading other men to believe, that the celestial sphere did actually perform theseovements.

olemy was an excellent geometer. He knew that the rising and the setting of the sun, theoon, and the myriad stars, could have been accounted for in a different way. If the earthrned round uniformly once a day while poised at the centre of the sphere of the heavens,




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e phenomena of rising and setting could be completely explained. This is, indeed, obviouster a moment's reflection. Consider yourself to be standing on the earth at the centre of teavens. There are stars over your head, and half the contents of the heavens are visible,hile the other half are below your horizon. As the earth turns round, the stars over your hll change, and unless it should happen that you have taken up your position at either of t

oles, new stars will pass into your view, and others will disappear, for at no time can youave more than half of the whole sphere visible. The observer on the earth would, therefor

y that some stars were rising, and that some stars were setting. We have, therefore, twotally distinct methods, each of which would completely explain all the observed facts of thurnal movement. One of these suppositions requires that the celestial sphere, bearing wite stars and other celestial bodies, turns uniformly around an invisible axis, while the earthmains stationary at the centre. The other supposition would be, that it is the stupendous

elestial sphere which remains stationary, while the earth at the centre rotates about the saxis as the celestial sphere did before, but in an opposite direction, and with a uniform velohich would enable it to complete one turn in twenty-four hours. Ptolemy was mathematicnough to know that either of these suppositions would suffice for the explanation of the

bserved facts. Indeed, the phenomena of the movements of the stars, so far as he couldbserve them, could not be called upon to pronounce which of these views was true, andhich was false.

olemy had, therefore, to resort for guidance to indirect lines of reasoning. One of theseppositions must be true, and yet it appeared that the adoption of either was accompanied

y a great difficulty. It is one of his chief merits to have demonstrated that the celestial sphas so stupendous that the earth itself was absolutely insignificant in comparison therewith then, this stupendous sphere rotated once in twenty-four hours, the speed with which th

ovement of some of the stars must be executed would be so portentous as to seem well-gh impossible. It would, therefore, seem much simpler on this ground to adopt the otherternative, and to suppose the diurnal movements were due to the rotation of the earth. Holemy saw, or at all events fancied he saw, objections of the weightiest description. The

vidence of the senses appeared directly to controvert the supposition that this earth isnything but stationary. Ptolemy might, perhaps, have dismissed this objection on the grouat the testimony of the senses on such a matter should be entirely subordinated to theterpretation which our intelligence would place upon the facts to which the senses deposenother objection, however, appeared to him to possess the gravest moment. It was argue

at if the earth were rotating, there is nothing to make the air participate in this motion,ankind would therefore be swept from the earth by the furious blasts which would arise fe movement of the earth through an atmosphere at rest. Even if we could imagine that thr were carried round with the earth, the same would not apply, so thought Ptolemy, to anbject suspended in the air. So long as a bird was perched on a tree, he might very well berried onward by the moving earth, but the moment he took wing, the ground would slipom under him at a frightful pace, so that when he dropped down again he would find hima distance perhaps ten times as great as that which a carrier-pigeon or a swallow could

ave traversed in the same time. Some vague delusion of this description seems even still t




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op up occasionally. I remember hearing of a proposition for balloon travelling of a verymarkable kind. The voyager who wanted to reach any other place in the same latitude wamply to ascend in a balloon, and wait there till the rotation of the earth conveyed the locahich happened to be his destination directly beneath him, whereupon he was to let out thas and drop down! Ptolemy knew quite enough natural philosophy to be aware that such aoposal for locomotion would be an utter absurdity; he knew that there was no such relatiift between the air and the earth as this motion would imply. It appeared to him to be

ecessary that the air should lag behind, if the earth had been animated by a movement oftation. In this he was, as we know, entirely wrong. There were, however, in his days no

ccurate notions on the subject of the laws of motion.

ssiduous as Ptolemy may have been in the study of the heavenly bodies, it seems evidentat he cannot have devoted much thought to the phenomena of motion of terrestrial objecmple, indeed, are the experiments which might have convinced a philosopher much lesscute than Ptolemy, that, if the earth did revolve, the air must necessarily accompany it. If der galloping on horseback tosses a ball into the air, it drops again into his hand, just as it

ould have done had he been remaining at rest during the ball's flight; the ball in factarticipates in the horizontal motion, so that though it really describes a curve as any passey would observe, yet it appears to the rider himself merely to move up and down in araight line. This fact, and many others similar to it, demonstrate clearly that if the earth wndowed with a movement of rotation, the atmosphere surrounding it must participate in thovement. Ptolemy did not know this, and consequently he came to the conclusion that th

arth did not rotate, and that, therefore, notwithstanding the tremendous improbability of sighty an object as the celestial sphere spinning round once in every twenty-four hours, thas no course open except to believe that this very improbable thing did really happen. Th

came to pass that Ptolemy adopted as the cardinal doctrine of his system a stationary earoised at the centre of the celestial sphere, which stretched around on all sides at a distanco vast that the diameter of the earth was an inappreciable point in comparison therewith.

olemy having thus deliberately rejected the doctrine of the earth's rotation, had to makeertain other entirely erroneous suppositions. It was easily seen that each star required exae same period for the performance of a complete revolution of the heavens. Ptolemy kneat the stars were at enormous distances from the earth, though no doubt his notions on t

oint came very far short of what we know to be the reality. If the stars had been at very

aried distances, then it would be so wildly improbable that they should all accomplish theirvolutions in the same time, that Ptolemy came to the conclusion that they must be all at tme distance, that is, that they must be all on the surface of a sphere. This view, howeverroneous, was corroborated by the obvious fact that the stars in the constellations preserveir relative places unaltered for centuries. Thus it was that Ptolemy came to the conclusioat they were all fixed on one spherical surface, though we are not informed as to theaterial of this marvellous setting which sustained the stars like jewels.

or should we hastily pronounce this doctrine to be absurd. The stars do appear to lie on th




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rface of a sphere, of which the observer is at the centre; not only is this the aspect whiche skies present to the untechnical observer, but it is the aspect in which the skies areesented to the most experienced astronomer of modern days. No doubt he knows well the stars are at the most varied distances from him; he knows that certain stars are ten tima hundred times, or a thousand times, as far as other stars. Nevertheless, to his eye the

ars appear on the surface of the sphere, it is on that surface that his measurements of thlative places of the stars are made; indeed, it may be said that almost all the accurate

bservations in the observatory relate to the places of the stars, not as they really are, but ey appear to be projected on that celestial sphere whose conception we owe to the geniuPtolemy.

his great philosopher shows very ingeniously that the earth must be at the centre of thephere. He proves that, unless this were the case, each star would not appear to move withe absolute uniformity which does, as a matter of fact, characterise it. In all these reasonine cannot but have the most profound admiration for the genius of Ptolemy, even though had made an error so enormous in the fundamental point of the stability of the earth. Anot

ror of a somewhat similar kind seemed to Ptolemy to be demonstrated. He had shown thae earth was an isolated object in space, and being such was, of course, capable of ovement. It could either be turned round, or it could be moved from one place to anothee know that Ptolemy deliberately adopted the view that the earth did not turn round; he en to investigate the other question, as to whether the earth was animated by anyovement of translation. He came to the conclusion that to attribute any motion to the earould be incompatible with the truths at which he had already arrived. The earth, arguedolemy, lies at the centre of the celestial sphere. If the earth were to be endowed withovement, it would not lie always at this point, it must, therefore, shift to some other part

e sphere. The movements of the stars, however, preclude the possibility of this; and,erefore, the earth must be as devoid of any movement of translation as it is devoid of tation. Thus it was that Ptolemy convinced himself that the stability of the earth, as it

ppeared to the ordinary senses, had a rational philosophical foundation.

ot unfrequently it is the lot of the philosophers to contend against the doctrines of theulgar, but when it happens, as in the case of Ptolemy's researches, that the doctrines of thulgar are corroborated by philosophical investigation which bear the stamp of the highestuthority, it is not to be wondered at that such doctrines should be deemed well-nigh

mpregnable. In this way we may, perhaps, account for the remarkable fact that the theoriePtolemy held unchallenged sway over the human intellect for the vast period already

entioned.

p to the present we have been speaking only of those primary motions of the heavens, byhich the whole sphere appeared to revolve once every twenty-four hours. We have now tscuss the remarkable theories by which Ptolemy endeavoured to account for the monthlyovement of the moon, for the annual movement of the sun, and for the periodic movemethe planets which had gained for them the titles of the wandering stars.




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anet always remains in the sun's vicinity. By properly proportioning the movements, this lontrivance simulated the transitions from the morning star to the evening star. Thus thehanges of Venus could be accounted for by a Combination of the "perfect" movement of Pe circle which it described uniformly round the earth, combined with the "perfect" motionenus in the circle which it described uniformly around the moving centre.

a precisely similar manner Ptolemy rendered an explanation of the fitful apparitions of

ercury. Now just on one side of the sun, and now just on the other, this rarely-seen planeoved like Venus on a circle whereof the centre was also carried by the line joining the sun

nd the earth. The circle, however, in which Mercury actually revolved had to be smaller that of Venus, in order to account for the fact that Mercury lies always much closer to the san the better-known planet.

IG. 2. PTOLEMY'S THEORY OF THE MOVEMENT OF MARS.]

he explanation of the movement of an outer planet like Mars could also be deduced from int effect of two perfect motions. The changes through which Mars goes are, however, sofferent from the movements of Venus that quite a different disposition of the circles isecessary. For consider the facts which characterise the movements of an outer planet suc Mars. In the first place, Mars accomplishes an entire circuit of the heaven. In this respec

o doubt, it may be said to resemble the sun or the moon. A little attention will, however,ow that there are extraordinary irregularities in the movement of the planet. Generally

peaking, it speeds its way from west to east among the stars, but sometimes the attentivebserver will note that the speed with which the planet advances is slackening, and then it em to become stationary. Some days later the direction of the planet's movement will be

versed, and it will be found moving from the east towards the west. At first it proceedsowly and then quickens its pace, until a certain speed is attained, which afterwards declinntil a second stationary position is reached. After a due pause the original motion from weeast is resumed, and is continued until a similar cycle of changes again commences. Suc

ovements as these were obviously quite at variance with any perfect movement in a singrcle round the earth. Here, again, the geometrical sagacity of Ptolemy provided him with teans of representing the apparent movements of Mars, and, at the same time, restrictinge explanation to those perfect movements which he deemed so essential. In Fig. 2 we

xhibit Ptolemy's theory as to the movement of Mars. We have, as before, the earth at the

entre, and the sun describing its circular orbit around that centre. The path of Mars is to bken as exterior to that of the sun. We are to suppose that at a point marked M there is actitious planet, which revolves around the earth uniformly, in a circle called the DEFERENThis point M, which is thus animated by a perfect movement, is the centre of a circle whichrried onwards with M, and around the circumference of which Mars revolves uniformly. It

asy to show that the combined effect of these two perfect movements is to produce exactat displacement of Mars in the heavens which observation discloses. In the positionpresented in the figure, Mars is obviously pursuing a course which will appear to the

bserver as a movement from west to east. When, however, the planet gets round to such




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osition as R, it is then moving from east to west in consequence of its revolution in theoving circle, as indicated by the arrowhead. On the other hand, the whole circle is carriedrward in the opposite direction. If the latter movement be less rapid than the former, thee shall have the backward movement of Mars on the heavens which it was desired toxplain. By a proper adjustment of the relative lengths of these arms the movements of theanet as actually observed could be completely accounted for.

he other outer planets with which Ptolemy was acquainted, namely, Jupiter and Saturn, haovements of the same general character as those of Mars. Ptolemy was equally successfu

xplaining the movements they performed by the supposition that each planet had perfecttation in a circle of its own, which circle itself had perfect movement around the earth in

entre.

is somewhat strange that Ptolemy did not advance one step further, as by so doing heould have given great simplicity to his system. He might, for instance, have represented tovements of Venus equally well by putting the centre of the moving circle at the sun itsel

nd correspondingly enlarging the circle in which Venus revolved. He might, too, haveranged that the several circles which the outer planets traversed should also have had th

entres at the sun. The planetary system would then have consisted of an earth fixed at theentre, of a sun revolving uniformly around it, and of a system of planets each describing itwn circle around a moving centre placed in the sun. Perhaps Ptolemy had not thought of t

perhaps he may have seen arguments against it. This important step was, however, takey Tycho. He considered that all the planets revolved around the sun in circles, and that then itself, bearing all these orbits, described a mighty circle around the earth. This point

aving been reached, only one more step would have been necessary to reach the glorious

uths that revealed the structure of the solar system. That last step was taken by Copernic

COPERNICUS.

he quaint town of Thorn, on the Vistula, was more than two centuries old when Copernicuas born there on the 19th of February, 1473. The situation of this town on the frontieretween Prussia and Poland, with the commodious waterway offered by the river, made it aace of considerable trade. A view of the town, as it was at the time of the birth of opernicus, is here given. The walls, with their watch-towers, will be noted, and the strateg

mportance which the situation of Thorn gave to it in the fifteenth century still belongs thereo much so that the German Government recently constituted the town a fortress of the firsass.

opernicus, the astronomer, whose discoveries make him the great predecessor of Kepler aewton, did not come from a noble family, as certain other early astronomers have done, fs father was a tradesman. Chroniclers are, however, careful to tell us that one of his uncleas a bishop. We are not acquainted with any of those details of his childhood or youth wh




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e often of such interest in other cases where men have risen to exalted fame. It wouldppear that the young Nicolaus, for such was his Christian name, received his education atome until such time as he was deemed sufficiently advanced to be sent to the University aracow. The education that he there obtained must have been in those days of a veryimitive description, but Copernicus seems to have availed himself of it to the utmost. He

evoted himself more particularly to the study of medicine, with the view of adopting itsactice as the profession of his life. The tendencies of the future astronomer were, howeve

vealed in the fact that he worked hard at mathematics, and, like one of his illustriousccessors, Galileo, the practice of the art of painting had for him a very great interest, andhe obtained some measure of success.

y the time he was twenty-seven years old, it would seem that Copernicus had given up thotion of becoming a medical practitioner, and had resolved to devote himself to science. Has engaged in teaching mathematics, and appears to have acquired some reputation. Hisowing fame attracted the notice of his uncle the bishop, at whose suggestion Copernicusok holy orders, and he was presently appointed to a canonry in the cathedral of Frauenbu

ear the mouth of the Vistula.

o Frauenburg, accordingly, this man of varied gifts retired. Possessing somewhat of thecetic spirit, he resolved to devote his life to work of the most serious description. Hechewed all ordinary society, restricting his intimacies to very grave and learned companio

nd refusing to engage in conversation of any useless kind. It would seem as if his gifts forainting were condemned as frivolous; at all events, we do not learn that he continued toactise them. In addition to the discharge of his theological duties, his life was occupied

artly in ministering medically to the wants of the poor, and partly with his researches in

tronomy and mathematics. His equipment in the matter of instruments for the study of theavens seems to have been of a very meagre description. He arranged apertures in the whis house at Allenstein, so that he could observe in some fashion the passage of the star

cross the meridian. That he possessed some talent for practical mechanics is proved by hionstruction of a contrivance for raising water from a stream, for the use of the inhabitantsauenburg. Relics of this machine are still to be seen.

he intellectual slumber of the Middle Ages was destined to be awakened by the revolutionoctrines of Copernicus. It may be noted, as an interesting circumstance, that the time at

hich he discovered the scheme of the solar system has coincided with a remarkable epoche world's history. The great astronomer had just reached manhood at the time whenolumbus discovered the new world.

efore the publication of the researches of Copernicus, the orthodox scientific creed averredat the earth was stationary, and that the apparent movements of the heavenly bodies wedeed real movements. Ptolemy had laid down this doctrine 1,400 years before. In his theois huge error was associated with so much important truth, and the whole presented such

oherent scheme for the explanation of the heavenly movements, that the Ptolemaic theory




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as not seriously questioned until the great work of Copernicus appeared. No doubt othersefore Copernicus, had from time to time in some vague fashion surmised, with more or lesausibility, that the sun, and not the earth, was the centre about which the system reallyvolved. It is, however, one thing to state a scientific fact; it is quite another thing to be in

ossession of the train of reasoning, founded on observation or experiment, by which that ay be established. Pythagoras, it appears, had indeed told his disciples that it was the sun

nd not the earth, which was the centre of movement, but it does not seem at all certain th

ythagoras had any grounds which science could recognise for the belief which is attributedm. So far as information is available to us, it would seem that Pythagoras associated hisheme of things celestial with a number of preposterous notions in natural philosophy. Heay certainly have made a correct statement as to which was the most important body in t

olar system, but he certainly did not provide any rational demonstration of the fact.opernicus, by a strict train of reasoning, convinced those who would listen to him that then was the centre of the system. It is useful for us to consider the arguments which heged, and by which he effected that intellectual revolution which is always connected with

ame.

he first of the great discoveries which Copernicus made relates to the rotation of the earths axis. That general diurnal movement, by which the stars and all other celestial bodiesppear to be carried completely round the heavens once every twenty-four hours, had beenccounted for by Ptolemy on the supposition that the apparent movements were the realovements. As we have already seen, Ptolemy himself felt the extraordinary difficulty involthe supposition that so stupendous a fabric as the celestial sphere should spin in the waypposed. Such movements required that many of the stars should travel with almostconceivable velocity. Copernicus also saw that the daily rising and setting of the heavenly

odies could be accounted for either by the supposition that the celestial sphere moved round that the earth remained at rest, or by the supposition that the celestial sphere was at rhile the earth turned round in the opposite direction. He weighed the arguments on bothdes as Ptolemy had done, and, as the result of his deliberations, Copernicus came to anpposite conclusion from Ptolemy. To Copernicus it appeared that the difficulties attending pposition that the celestial sphere revolved, were vastly greater than those which appear

o weighty to Ptolemy as to force him to deny the earth's rotation.

opernicus shows clearly how the observed phenomena could be accounted for just as

ompletely by a rotation of the earth as by a rotation of the heavens. He alludes to the factat, to those on board a vessel which is moving through smooth water, the vessel itself

ppears to be at rest, while the objects on shore seem to be moving past. If, therefore, thearth were rotating uniformly, we dwellers upon the earth, oblivious of our own movement,ould wrongly attribute to the stars the displacement which was actually the consequence ur own motion.

opernicus saw the futility of the arguments by which Ptolemy had endeavoured toemonstrate that a revolution of the earth was impossible. It was plain to him that there w




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othing whatever to warrant refusal to believe in the rotation of the earth. In his clear-ghtedness on this matter we have specially to admire the sagacity of Copernicus as a natuhilosopher. It had been urged that, if the earth moved round, its motion would not be

mparted to the air, and that therefore the earth would be uninhabitable by the terrific windhich would be the result of our being carried through the air. Copernicus convinced himseat this deduction was preposterous. He proved that the air must accompany the earth, ju his coat remains round him, notwithstanding the fact that he is walking down the street.

is way he was able to show that all a priori objections to the earth's movements werebsurd, and therefore he was able to compare together the plausibilities of the two rivalhemes for explaining the diurnal movement.

nce the issue had been placed in this form, the result could not be long in doubt. Here is tuestion: Which is it more likely-- that the earth, like a grain of sand at the centre of a migobe, should turn round once in twenty-four hours, or that the whole of that vast globeould complete a rotation in the opposite direction in the same time? Obviously, the former the more simple supposition. But the case is really much stronger than this. Ptolemy had

pposed that all the stars were attached to the surface of a sphere. He had no groundhatever for this supposition, except that otherwise it would have been well-nigh impossiblhave devised a scheme by which the rotation of the heavens around a fixed earth could

ave been arranged. Copernicus, however, with the just instinct of a philosopher, considereat the celestial sphere, however convenient from a geometrical point of view, as a meanspresenting apparent phenomena, could not actually have a material existence. In the firstace, the existence of a material celestial sphere would require that all the myriad starsould be at exactly the same distances from the earth. Of course, no one will say that this

ny other arbitrary disposition of the stars is actually impossible, but as there was no

onceivable physical reason why the distances of all the stars from the earth should beentical, it seemed in the very highest degree improbable that the stars should be so place

oubtless, also, Copernicus felt a considerable difficulty as to the nature of the materials frohich Ptolemy's wonderful sphere was to be constructed. Nor could a philosopher of hisenetration have failed to observe that, unless that sphere were infinitely large, there mustave been space outside it, a consideration which would open up other difficult questions.hether infinite or not, it was obvious that the celestial sphere must have a diameter at leaany thousands of times as great as that of the earth. From these considerations Copernic

educed the important fact that the stars and the other celestial bodies must all be vastbjects. He was thus enabled to put the question in such a form that it could hardly receiveny answer but the correct one. Which is it more rational to suppose, that the earth shouldrn round on its axis once in twenty-four hours, or that thousands of mighty stars shouldrcle round the earth in the same time, many of them having to describe circles manyousands of times greater in circumference than the circuit of the earth at the equator? Th

bvious answer pressed upon Copernicus with so much force that he was compelled to rejeolemy's theory of the stationary earth, and to attribute the diurnal rotation of the heavene revolution of the earth on its axis.




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nce this tremendous step had been taken, the great difficulties which beset the monstrouonception of the celestial sphere vanished, for the stars need no longer be regarded astuated at equal distances from the earth. Copernicus saw that they might lie at the mostaried degrees of remoteness, some being hundreds or thousands of times farther away thhers. The complicated structure of the celestial sphere as a material object disappearedtogether; it remained only as a geometrical conception, whereon we find it convenient to

dicate the places of the stars. Once the Copernican doctrine had been fully set forth, it wampossible for anyone, who had both the inclination and the capacity to understand it, to

thhold acceptance of its truth. The doctrine of a stationary earth had gone for ever.

opernicus having established a theory of the celestial movements which deliberately set ase stability of the earth, it seemed natural that he should inquire whether the doctrine of aoving earth might not remove the difficulties presented in other celestial phenomena. It heen universally admitted that the earth lay unsupported in space. Copernicus had furtherown that it possessed a movement of rotation. Its want of stability being thus recognised

emed reasonable to suppose that the earth might also have some other kinds of moveme well. In this, Copernicus essayed to solve a problem far more difficult than that which hatherto occupied his attention. It was a comparatively easy task to show how the diurnalsing and setting could be accounted for by the rotation of the earth. It was a much morefficult undertaking to demonstrate that the planetary movements, which Ptolemy hadpresented with so much success, could be completely explained by the supposition that ethose planets revolved uniformly round the sun, and that the earth was also a planet,

ccomplishing a complete circuit of the sun once in the course of a year.

would be impossible in a sketch like the present to enter into any detail as to theeometrical propositions on which this beautiful investigation of Copernicus depended. We nly mention a few of the leading principles. It may be laid down in general that, if anbserver is in movement, he will, if unconscious of the fact, attribute to the fixed objectsound him a movement equal and opposite to that which he actually possesses. A passeng

n a canal-boat sees the objects on the banks apparently moving backward with a speedqual to that by which he is himself advancing forwards. By an application of this principle, n account for all the phenomena of the movements of the planets, which Ptolemy had sogeniously represented by his circles. Let us take, for instance, the most characteristic feat

the irregularities of the outer planets. We have already remarked that Mars, thoughenerally advancing from west to east among the stars, occasionally pauses, retraces his str awhile, again pauses, and then resumes his ordinary onward progress. Copernicus showearly how this effect was produced by the real motion of the earth, combined with the reaotion of Mars. In the adjoining figure we represent a portion of the circular tracks in whice earth and Mars move in accordance with the Copernican doctrine. I show particularly thse where the earth comes directly between the planet and the sun, because it is on such

ccasions that the retrograde movement (for so this backward movement of Mars is termedits highest. Mars is then advancing in the direction shown by the arrow-head, and the ea




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also advancing in the same direction. We, on the earth, however, being unconscious of own motion, attribute, by the principle I have already explained, an equal and opposite mot

Mars. The visible effect upon the planet is, that Mars has two movements, a real onwardovement in one direction, and an apparent movement in the opposite direction. If it soappened that the earth was moving with the same speed as Mars, then the apparentovement would exactly neutralise the real movement, and Mars would seem to be at restlatively to the surrounding stars. Under the actual circumstances represented, however, t

arth is moving faster than Mars, and the consequence is, that the apparent movement of tanet backwards exceeds the real movement forwards, the net result being an apparenttrograde movement.

ith consummate skill, Copernicus showed how the applications of the same principles couccount for the characteristic movements of the planets. His reasoning in due time bore do opposition. The supreme importance of the earth in the system vanished. It had nowerely to take rank as one of the planets.

he same great astronomer now, for the first time, rendered something like a rational accothe changes of the seasons. Nor did certain of the more obscure astronomical phenomencape his attention.

e delayed publishing his wonderful discoveries to the world until he was quite an old mane had a well-founded apprehension of the storm of opposition which they would arouse.owever, he yielded at last to the entreaties of his friends, and his book was sent to the prut ere it made its appearance to the world, Copernicus was seized by mortal illness. A copthe book was brought to him on May 23, 1543. We are told that he was able to see it an

touch it, but no more, and he died a few hours afterwards. He was buried in that CathedFrauenburg, with which his life had been so closely associated.

TYCHO BRAHE.

he most picturesque figure in the history of astronomy is undoubtedly that of the famous anish astronomer whose name stands at the head of this chapter. Tycho Brahe was alikeotable for his astronomical genius and for the extraordinary vehemence of a character whas by no means perfect. His romantic career as a philosopher, and his taste for splendour

Danish noble, his ardent friendships and his furious quarrels, make him an ideal subject foographer, while the magnificent astronomical work which he accomplished, has given him

mperishable fame.

he history of Tycho Brahe has been admirably told by Dr. Dreyer, the accomplishedtronomer who now directs the observatory at Armagh, though himself a countryman of

ycho. Every student of the career of the great Dane must necessarily look on Dr. Dreyer'sork as the chief authority on the subject. Tycho sprang from an illustrious stock. His famil




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ad flourished for centuries, both in Sweden and in Denmark, where his descendants are toet with at the present day. The astronomer's father was a privy councillor, and having fille

mportant positions in the Danish government, he was ultimately promoted to be governor elsingborg Castle, where he spent the last years of his life. His illustrious son Tycho was b1546, and was the second child and eldest boy in a family of ten.

appears that Otto, the father of Tycho, had a brother named George, who was childless.

eorge, however, desired to adopt a boy on whom he could lavish his affection and to whoe could bequeath his wealth. A somewhat singular arrangement was accordingly entered iy the brothers at the time when Otto was married. It was agreed that the first son who me born to Otto should be forthwith handed over by the parents to George to be reared anddopted by him. In due time little Tycho appeared, and was immediately claimed by Georgeursuance of the compact. But it was not unnatural that the parental instinct, which had beormant when the agreement was made, should here interpose. Tycho's father and motherceded from the bargain, and refused to part with their son. George thought he was badlyeated. However, he took no violent steps until a year later, when a brother was born to

ycho. The uncle then felt no scruple in asserting what he believed to be his rights by themple process of stealing the first-born nephew, which the original bargain had promised hter a little time it would seem that the parents acquiesced in the loss, and thus it was inncle George's home that the future astronomer passed his childhood.

hen we read that Tycho was no more than thirteen years old at the time he entered theniversity of Copenhagen, it might be at first supposed that even in his boyish years he muave exhibited some of those remarkable talents with which he was afterwards to astonish orld. Such an inference should not, however, be drawn. The fact is that in those days it w

ustomary for students to enter the universities at a much earlier age than is now the case.ot, indeed, that the boys of thirteen knew more then than the boys of thirteen know now.ut the education imparted in the universities at that time was of a much more rudimentarynd than that which we understand by university education at present. In illustration of thir. Dreyer tells us how, in the University of Wittenberg, one of the professors, in his openinddress, was accustomed to point out that even the processes of multiplication and divisionithmetic might be learned by any student who possessed the necessary diligence.

was the wish and the intention of his uncle that Tycho's education should be specially

rected to those branches of rhetoric and philosophy which were then supposed to be aecessary preparation for the career of a statesman. Tycho, however, speedily made it plais teachers that though he was an ardent student, yet the things which interested him wee movements of the heavenly bodies and not the subtleties of metaphysics.

n the 21st October, 1560, an eclipse of the sun occurred, which was partially visible atopenhagen. Tycho, boy though he was, took the utmost interest in this event. His ardour tonishment in connection with the circumstance were chiefly excited by the fact that the

me of the occurrence of the phenomenon could be predicted with so much accuracy. Urge




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y his desire to understand the matter thoroughly, Tycho sought to procure some book whight explain what he so greatly wanted to know. In those days books of any kind were buw and scarce, and scientific books were especially unattainable. It so happened, howeverat a Latin version of Ptolemy's astronomical works had appeared a few years before the

clipse took place, and Tycho managed to buy a copy of this book, which was then the chieuthority on celestial matters. Young as the boy astronomer was, he studied hard, althougherhaps not always successfully, to understand Ptolemy, and to this day his copy of the gre

ork, copiously annotated and marked by the schoolboy hand, is preserved as one of the ceasures in the library of the University at Prague.

ter Tycho had studied for about three years at the University of Copenhagen, his uncleought it would be better to send him, as was usual in those days, to complete his educat

y a course of study in some foreign university. The uncle cherished the hope that in this we attention of the young astronomer might be withdrawn from the study of the stars andrected in what appeared to him a more useful way. Indeed, to the wise heads of those dae pursuit of natural science seemed so much waste of good time which might otherwise b

evoted to logic or rhetoric or some other branch of study more in vogue at that time. Tosist in this attempt to wean Tycho from his scientific tastes, his uncle chose as a tutor to

ccompany him an intelligent and upright young man named Vedel, who was four years senhis pupil, and accordingly, in 1562, we find the pair taking up their abode at the UniversiLeipzig.

he tutor, however, soon found that he had undertaken a most hopeless task. He could nocceed in imbuing Tycho with the slightest taste for the study of the law or the otheranches of knowledge which were then thought so desirable. The stars, and nothing but th

ars, engrossed the attention of his pupil. We are told that all the money he could obtain wpent secretly in buying astronomical books and instruments. He learned the name of the som a little globe, which he kept hidden from Vedel, and only ventured to use during thetter's absence. No little friction was at first caused by all this, but in after years a fast andnduring friendship grew up between Tycho and his tutor, each of whom learned to respecnd to love the other.

efore Tycho was seventeen he had commenced the difficult task of calculating theovements of the planets and the places which they occupied on the sky from time to time

e was not a little surprised to find that the actual positions of the planets differed very widom those which were assigned to them by calculations from the best existing works of tronomers. With the insight of genius he saw that the only true method of investigating tovements of the heavenly bodies would be to carry on a protracted series of measurementheir places. This, which now seems to us so obvious, was then entirely new doctrine.

ycho at once commenced regular observations in such fashion as he could. His firststrument was, indeed, a very primitive one, consisting of a simple pair of compasses, whie used in this way. He placed his eye at the hinge, and then opened the legs of the compao that one leg pointed to one star and the other leg to the other star. The compass was th




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ought down to a divided circle, by which means the number of degrees in the apparentngular distance of the two stars was determined.

s next advance in instrumental equipment was to provide himself with the contrivancenown as the "cross-staff," which he used to observe the stars whenever opportunity offeremust, of course, be remembered that in those days there were no telescopes. In the

bsence of optical aid, such as lenses afford the modern observers, astronomers had to rely

n mechanical appliances alone to measure the places of the stars. Of such appliances,erhaps the most ingenious was one known before Tycho's time, which we have representethe adjoining figure.

et us suppose that it be desired to measure the angle between two stars, then if the angleot too large it can be determined in the following manner. Let the rod AB be divided intoches and parts of an inch, and let another rod, CD, slide up and down along AB in such aay that the two always remain perpendicular to each other. "Sights," like those on a rifle, aced at A and C, and there is a pin at D. It will easily be seen that, by sliding the movable

ar along the fixed one, it must always be possible when the stars are not too far apart toing the sights into such positions that one star can be seen along DC and the other alongA. This having been accomplished, the length from A to the cross-bar is read off on theale, and then, by means of a table previously prepared, the value of the required angularstance is obtained. If the angle between the two stars were greater than it would be possmeasure in the way already described, then there was a provision by which the pin at D

ight be moved along CD into some other position, so as to bring the angular distance of tars within the range of the instrument.

o doubt the cross-staff is a very primitive contrivance, but when handled by one so skilful ycho it afforded results of considerable accuracy. I would recommend any reader who maave a taste for such pursuits to construct a cross-staff for himself, and see whateasurements he can accomplish with its aid.

o employ this little instrument Tycho had to evade the vigilance of his conscientious tutor,ho felt it his duty to interdict all such occupations as being a frivolous waste of time. It wahen Vedel was asleep that Tycho managed to escape with his cross staff and measure theaces of the heavenly bodies. Even at this early age Tycho used to conduct his observation

n those thoroughly sound principles which lie at the foundation of all accurate moderntronomy. Recognising the inevitable errors of workmanship in his little instrument, hecertained their amount and allowed for their influence on the results which he deduced. Tinciple, employed by the boy with his cross-staff in 1564, is employed at the present day e Astronomer Royal at Greenwich with the most superb instruments that the skill of mode

pticians has been able to construct.

ter the death of his uncle, when Tycho was nineteen years of age, it appears that the you




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hilosopher was no longer interfered with in so far as the line which his studies were to takas concerned. Always of a somewhat restless temperament, we now find that he shifted hbode to the University of Rostock, where he speedily made himself notable in connection wn eclipse of the moon on 28th October, 1566. Like every other astronomer of those days,ycho had always associated astronomy with astrology. He considered that the phenomenae heavenly bodies always had some significance in connection with human affairs. Tychoas also a poet, and in the united capacity of poet, astrologer, and astronomer, he posted

ome verses in the college at Rostock announcing that the lunar eclipse was a prognosticatthe death of the great Turkish Sultan, whose mighty deeds at that time filled men's mindesently news did arrive of the death of the Sultan, and Tycho was accordingly triumphant

ut a little later it appeared that the decease had taken place BEFORE the eclipse, arcumstance which caused many a laugh at Tycho's expense.

ycho being of a somewhat turbulent disposition, it appears that, while at the University of ostock, he had a serious quarrel with another Danish nobleman. We are not told for certahat was the cause of the dispute. It does not, however, seem to have had any more

mantic origin than a difference of opinion as to which of them knew the more mathematihey fought, as perhaps it was becoming for two astronomers to fight, under the canopy oeaven in utter darkness at the dead of night, and the duel was honourably terminated wheslice was taken off Tycho's nose by the insinuating sword of his antagonist. For the repairis injury the ingenuity of the great instrument-maker was here again useful, and he madebstitute for his nose "with a composition of gold and silver." The imitation was so good this declared to have been quite equal to the original. Dr. Lodge, however, pointedly observat it does not appear whether this remark was made by a friend or an enemy.

he next few years Tycho spent in various places ardently pursuing somewhat varied brancscientific study. At one time we hear of him assisting an astronomical alderman, in the

ncient city of Augsburg, to erect a tremendous wooden machine--a quadrant of 19-feetdius--to be used in observing the heavens. At another time we learn that the King of enmark had recognised the talents of his illustrious subject, and promised to confer on himeasant sinecure in the shape of a canonry, which would assist him with the means fordulging his scientific pursuits. Again we are told that Tycho is pursuing experiments in

hemistry with the greatest energy, nor is this so incompatible as might at first be thoughtth his devotion to astronomy. In those early days of knowledge the different sciences

emed bound together by mysterious bonds. Alchemists and astrologers taught that theveral planets were correlated in some mysterious manner with the several metals. It waserefore hardly surprising that Tycho should have included a study of the properties of theetals in the programme of his astronomical work.

n event, however, occurred in 1572 which stimulated Tycho's astronomical labours, andarted him on his life's work. On the 11th of November in that year, he was returning homsupper after a day's work in his laboratory, when he happened to lift his face to the sky,

nd there he beheld a brilliant new star. It was in the constellation of Cassiopeia, and




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ccupied a position in which there had certainly been no bright star visible when his attentiad last been directed to that part of the heavens. Such a phenomenon was so startling thae found it hard to trust the evidence of his senses. He thought he must be the subject of ome hallucination. He therefore called to the servants who were accompanying him, andked them whether they, too, could see a brilliant object in the direction in which he point

hey certainly could, and thus he became convinced that this marvellous object was no meeation of the fancy, but a veritable celestial body--a new star of surpassing splendour wh

ad suddenly burst forth. In these days of careful scrutiny of the heavens, we are accustomthe occasional outbreak of new stars. It is not, however, believed that any new star whic

as ever appeared has displayed the same phenomenal brilliance as was exhibited by the s1572.

his object has a value in astronomy far greater than it might at first appear. It is true, in onse, that Tycho discovered the new star, but it is equally true, in a different sense, that itas the new star which discovered Tycho. Had it not been for this opportune apparition, it uite possible that Tycho might have found a career in some direction less beneficial to

ience than that which he ultimately pursued.

hen he reached his home on this memorable evening, Tycho immediately applied his greauadrant to the measurement of the place of the new star. His observations were speciallyrected to the determination of the distance of the object. He rightly conjectured that if itere very much nearer to us than the stars in its vicinity, the distance of the brilliant bodyight be determined in a short time by the apparent changes in its distance from therrounding points. It was speedily demonstrated that the new star could not be as near ase moon, by the simple fact that its apparent place, as compared with the stars in its

eighbourhood, was not appreciably altered when it was observed below the pole, and agabove the pole at an interval of twelve hours. Such observations were possible, inasmuch ae star was bright enough to be seen in full daylight. Tycho thus showed conclusively that

ody was so remote that the diameter of the earth bore an insignificant ratio to the star'sstance. His success in this respect is the more noteworthy when we find that many otherbservers, who studied the same object, came to the erroneous conclusion that the new staas quite as near as the moon, or even much nearer. In fact, it may be said, that with regathis object Tycho discovered everything which could possibly have been discovered in th

ays before telescopes were invented. He not only proved that the star's distance was too

eat for measurement, but he showed that it had no proper motion on the heavens. Hecorded the successive changes in its brightness from week to week, as well as the

uctuations in hue with which the alterations in lustre were accompanied.

seems, nowadays, strange to find that such thoroughly scientific observations of the newar as those which Tycho made, possessed, even in the eyes of the great astronomer himsprofound astrological significance. We learn from Dr. Dreyer that, in Tycho's opinion, "thear was at first like Venus and Jupiter, and its effects will therefore, first, be pleasant; but then became like Mars, there will next come a period of wars, seditions, captivity, and de




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princes, and destruction of cities, together with dryness and fiery meteors in the air,estilence, and venomous snakes. Lastly, the star became like Saturn, and thus will finallyome a time of want, death, imprisonment, and all kinds of sad things!" Ideas of this kindere, however, universally entertained. It seemed, indeed, obvious to learned men of thateriod that such an apparition must forebode startling events. One of the chief theories theeld was, that just as the Star of Bethlehem announced the first coming of Christ, so thecond coming, and the end of the world, was heralded by the new star of 1572.

he researches of Tycho on this object were the occasion of his first appearance as an authhe publication of his book was however, for some time delayed by the urgent remonstranchis friends, who thought it was beneath the dignity of a nobleman to condescend to writ

ook. Happily, Tycho determined to brave the opinion of his order; the book appeared, andas the first of a series of great astronomical productions from the same pen.

he fame of the noble Dane being now widespread, the King of Denmark entreated him toturn to his native country, and to deliver a course of lectures on astronomy in the Univers

Copenhagen. With some reluctance he consented, and his introductory oration has beeneserved. He dwells, in fervent language, upon the beauty and the interest of the celestial

henomena. He points out the imperative necessity of continuous and systematic observatithe heavenly bodies in order to extend our knowledge. He appeals to the practical utility e science, for what civilised nation could exist without having the means of measuring time sets forth how the study of these beautiful objects "exalts the mind from earthly and trivings to heavenly ones;" and then he winds up by assuring them that a special use of tronomy is that it enables us to draw conclusions from the movements in the celestialgions as to human fate."

n interesting event, which occurred in 1572, distracted Tycho's attention from astronomicaatters. He fell in love. The young girl on whom his affections were set appears to have

prung from humble origin. Here again his august family friends sought to dissuade him fromatch they thought unsuitable for a nobleman. But Tycho never gave way in anything. Itggested that he did not seek a wife among the highborn dames of his own rank from theead that the demands of a fashionable lady would make too great an inroad on the timeat he wished to devote to science. At all events, Tycho's union seems to have been a hap

ne, and he had a large family of children; none of whom, however, inherited their father's

lents.

ycho had many scientific friends in Germany, among whom his work was held in highteem. The treatment that he there met with seemed to him so much more encouraging tat which he received in Denmark that he formed the notion of emigrating to Basle andaking it his permanent abode. A whisper of this intention was conveyed to the large-heartng of Denmark, Frederick II. He wisely realised how great would be the fame which woulccrue to his realm if he could induce Tycho to remain within Danish territory and carry onere the great work of his life. A resolution to make a splendid proposal to Tycho was




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mmediately formed. A noble youth was forthwith despatched as a messenger, and orderedavel day and night until he reached Tycho, whom he was to summon to the king. Thetronomer was in bed on the morning Of 11th February, 1576, when the message was

elivered. Tycho, of course, set off at once and had an audience of the king at Copenhagenhe astronomer explained that what he wanted was the means to pursue his studiesnmolested, whereupon the king offered him the Island of Hven, in the Sound near Elsinorehere he would enjoy all the seclusion that he could desire. The king further promised that

ould provide the funds necessary for building a house and for founding the greatestbservatory that had ever yet been reared for the study of the heavens. After due deliberatnd consultation with his friends, Tycho accepted the king's offer. He was forthwith grantedension, and a deed was drawn up formally assigning the Island of Hven to his use all theays of his life.

he foundation of the famous castle of Uraniborg was laid on 30th August, 1576. Theeremony was a formal and imposing one, in accordance with Tycho's ideas of splendour. Aarty of scientific friends had assembled, and the time had been chosen so that the heaven

odies were auspiciously placed. Libations of costly wines were poured forth, and the stoneas placed with due solemnity. The picturesque character of this wonderful temple for theudy of the stars may be seen in the figures with which this chapter is illustrated.

ne of the most remarkable instruments that has ever been employed in studying the heavas the mural quadrant which Tycho erected in one of the apartments of Uraniborg. By itseans the altitudes of the celestial bodies could be observed with much greater accuracy thad been previously attainable. This wonderful contrivance is represented on the precedingage. It will be observed that the walls of the room are adorned by pictures with a lavishne

decoration not usually to be found in scientific establishments.

few years later, when the fame of the observatory at Hven became more widely spread, aumber of young men flocked to Tycho to study under his direction. He therefore built anotbservatory for their use in which the instruments were placed in subterranean rooms of hich only the roofs appeared above the ground. There was a wonderful poetical inscriptiover the entrance to this underground observatory, expressing the astonishment of Urania nding, even in the interior of the earth, a cavern devoted to the study of the heavens. Tycas indeed always fond of versifying, and he lost no opportunity of indulging this taste

henever an occasion presented itself.

ound the walls of the subterranean observatory were the pictures of eight astronomers,ach with a suitable inscription--one of these of course represented Tycho himself, andeneath were written words to the effect that posterity should judge of his work. The eightcture depicted an astronomer who has not yet come into existence. Tychonides was hisame, and the inscription presses the modest hope that when he does appear he will beorthy of his great predecessor. The vast expenses incurred in the erection and theaintenance of this strange establishment were defrayed by a succession of grants from th




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yal purse.

or twenty years Tycho laboured hard at Uraniborg in the pursuit of science. His work mainonsisted in the determination of the places of the moon, the planets, and the stars on theelestial sphere. The extraordinary pains taken by Tycho to have his observations as accura his instruments would permit, have justly entitled him to the admiration of all succeedingtronomers. His island home provided the means of recreation as well as a place for work.

as surrounded by his family, troops of friends were not wanting, and a pet dwarf seems toave been an inmate of his curious residence. By way of change from his astronomical laboe used frequently to work with his students in his chemical laboratory. It is not indeed knohat particular problems in chemistry occupied his attention. We are told, however, that hengaged largely in the production of medicines, and as these appear to have been dispenseatuitously there was no lack of patients.

ycho's imperious and grasping character frequently brought him into difficulties, which seehave increased with his advancing years. He had ill-treated one of his tenants on Hven, a

n adverse decision by the courts seems to have greatly exasperated the astronomer. Seriohanges also took place in his relations to the court at Copenhagen. When the young king wowned in 1596, he reversed the policy of his predecessor with reference to Hven. The libeowances to Tycho were one after another withdrawn, and finally even his pension wasopped. Tycho accordingly abandoned Hven in a tumult of rage and mortification. A few yeter we find him in Bohemia a prematurely aged man, and he died on the 24th October,601.

GALILEO.

mong the ranks of the great astronomers it would be difficult to find one whose life preseore interesting features and remarkable vicissitudes than does that of Galileo. We may

onsider him as the patient investigator and brilliant discoverer. We may consider him in hisivate relations, especially to his daughter, Sister Maria Celeste, a woman of very remarka

haracter; and we have also the pathetic drama at the close of Galileo's life, when thehilosopher drew down upon himself the thunders of the Inquisition.

he materials for the sketch of this astonishing man are sufficiently abundant. We makepecial use in this place of those charming letters which his daughter wrote to him from heonvent home. More than a hundred of these have been preserved, and it may well beoubted whether any more beautiful and touching series of letters addressed to a parent byearly loved child have ever been written. An admirable account of this correspondence isontained in a little book entitled "The Private Life of Galileo," published anonymously byessrs. Macmillan in 1870, and I have been much indebted to the author of that volume foany of the facts contained in this chapter.




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alileo was born at Pisa, on 18th February, 1564. He was the eldest son of Vincenzo de'onajuti de' Galilei, a Florentine noble. Notwithstanding his illustrious birth and descent, itould seem that the home in which the great philosopher's childhood was spent was an

mpoverished one. It was obvious at least that the young Galileo would have to be providedth some profession by which he might earn a livelihood. From his father he derived both heritance and by precept a keen taste for music, and it appears that he became an excellerformer on the lute. He was also endowed with considerable artistic power, which he

ultivated diligently. Indeed, it would seem that for some time the future astronomerntertained the idea of devoting himself to painting as a profession. His father, however,ecided that he should study medicine. Accordingly, we find that when Galileo was seventeears of age, and had added a knowledge of Greek and Latin to his acquaintance with the fts, he was duly entered at the University of Pisa.

ere the young philosopher obtained some inkling of mathematics, whereupon he became uch interested in this branch of science, that he begged to be allowed to study geometry.

ompliance with his request, his father permitted a tutor to be engaged for this purpose; bu

e did so with reluctance, fearing that the attention of the young student might thus bethdrawn from that medical work which was regarded as his primary occupation. The even

peedily proved that these anxieties were not without some justification. The propositions ouclid proved so engrossing to Galileo that it was thought wise to avoid further distraction brminating the mathematical tutor's engagement. But it was too late for the desired end totained. Galileo had now made such progress that he was able to continue his geometricaludies by himself. Presently he advanced to that famous 47th proposition which won his livdmiration, and on he went until he had mastered the six books of Euclid, which was aonsiderable achievement for those days.

he diligence and brilliance of the young student at Pisa did not, however, bring him muchedit with the University authorities. In those days the doctrines of Aristotle were regardede embodiment of all human wisdom in natural science as well as in everything else. It wagarded as the duty of every student to learn Aristotle off by heart, and any disposition to

oubt or even to question the doctrines of the venerated teacher was regarded as intolerabesumption. But young Galileo had the audacity to think for himself about the laws of natue would not take any assertion of fact on the authority of Aristotle when he had the meanquestioning nature directly as to its truth or falsehood. His teachers thus came to regard

m as a somewhat misguided youth, though they could not but respect the unflaggingdustry with which he amassed all the knowledge he could acquire.

e are so accustomed to the use of pendulums in our clocks that perhaps we do not oftenalise that the introduction of this method of regulating time-pieces was really a notablevention worthy the fame of the great astronomer to whom it was due. It appears that sittne day in the Cathedral of Pisa, Galileo's attention became concentrated on the swinging ohandelier which hung from the ceiling. It struck him as a significant point, that whether thc through which the pendulum oscillated was a long one or a short one, the time occupie




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ach vibration was sensibly the same. This suggested to the thoughtful observer that aendulum would afford the means by which a time-keeper might be controlled, andccordingly Galileo constructed for the first time a clock on this principle. The immediatebject sought in this apparatus was to provide a means of aiding physicians in counting theulses of their patients.

he talents Of Galileo having at length extorted due recognition from the authorities, he wa

ppointed, at the age of twenty-five, Professor of Mathematics at the University of Pisa. Thme the time when he felt himself strong enough to throw down the gauntlet to the

dherents of the old philosophy. As a necessary part of his doctrine on the movement of odies Aristotle had asserted that the time occupied by a stone in falling depends upon itseight, so that the heavier the stone the less time would it require to fall from a certain hethe earth. It might have been thought that a statement so easily confuted by the simples

xperiments could never have maintained its position in any accepted scheme of philosophyut Aristotle had said it, and to anyone who ventured to express a doubt the ready sneer wrthcoming, "Do you think yourself a cleverer man than Aristotle?" Galileo determined to

emonstrate in the most emphatic manner the absurdity of a doctrine which had for centurceived the sanction of the learned. The summit of the Leaning Tower of Pisa offered a higamatic site for the great experiment. The youthful professor let fall from the overhangingp a large heavy body and a small light body simultaneously. According to Aristotle the lar

ody ought to have reached the ground much sooner than the small one, but such was fouot to be the case. In the sight of a large concourse of people the simple fact wasemonstrated that the two bodies fell side by side, and reached the ground at the same timhus the first great step was taken in the overthrow of that preposterous system of nquestioning adhesion to dogma, which had impeded the development of the knowledge o

ature for nearly two thousand years.

his revolutionary attitude towards the ancient beliefs was not calculated to render Galileo'slations with the University authorities harmonious. He had also the misfortune to make

nemies in other quarters. Don Giovanni de Medici, who was then the Governor of the Porteghorn, had designed some contrivance by which he proposed to pump out a dock. Butalileo showed up the absurdity of this enterprise in such an aggressive manner that Donovanni took mortal offence, nor was he mollified when the truths of Galileo's criticisms we

bundantly verified by the total failure of his ridiculous invention. In various ways Galileo w

ade to feel his position at Pisa so unpleasant that he was at length compelled to abandonhair in the University. The active exertions of his friends, of whom Galileo was so fortunate

have had throughout his life an abundant supply, then secured his election to theofessorship of Mathematics at Padua, whither he went in 1592.

was in this new position that Galileo entered on that marvellous career of investigationhich was destined to revolutionize science. The zeal with which he discharged hisofessorial duties was indeed of the most unremitting character. He speedily drew suchowds to listen to his discourses on Natural Philosophy that his lecture-room was filled to




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verflowing. He also received many private pupils in his house for special instruction. Everyoment that could be spared from these labours was devoted to his private study and to hcessant experiments.

ke many another philosopher who has greatly extended our knowledge of nature, Galileoad a remarkable aptitude for the invention of instruments designed for philosophicalsearch. To facilitate his practical work, we find that in 1599 he had engaged a skilled

orkman who was to live in his house, and thus be constantly at hand to try the devices fover springing from Galileo's fertile brain. Among the earliest of his inventions appears to heen the thermometer, which he constructed in 1602. No doubt this apparatus in its primitirm differed in some respects from the contrivance we call by the same name. Galileo at f

mployed water as the agent, by the expansion of which the temperature was to beeasured. He afterwards saw the advantage of using spirits for the same purpose. It was nntil about half a century later that mercury came to be recognised as the liquid mostenerally suitable for the thermometer.

he time was now approaching when Galileo was to make that mighty step in thedvancement of human knowledge which followed on the application of the telescope totronomy. As to how his idea of such an instrument originated, we had best let him tell uss own words. The passage is given in a letter which he writes to his brother-in-law,anducci.

write now because I have a piece of news for you, though whether you will be glad or sohear it I cannot say; for I have now no hope of returning to my own country, though the

ccurrence which has destroyed that hope has had results both useful and honourable. You

ust know, then, that two months ago there was a report spread here that in Flanders somne had presented to Count Maurice of Nassau a glass manufactured in such a way as toake distant objects appear very near, so that a man at the distance of two miles could beearly seen. This seemed to me so marvellous that I began to think about it. As it appearede to have a foundation in the Theory of Perspective, I set about contriving how to make i

nd at length I found out, and have succeeded so well that the one I have made is farperior to the Dutch telescope. It was reported in Venice that I had made one, and a wee

nce I was commanded to show it to his Serenity and to all the members of the senate, toeir infinite amazement. Many gentlemen and senators, even the oldest, have ascended at

arious times the highest bell-towers in Venice to spy out ships at sea making sail for theouth of the harbour, and have seen them clearly, though without my telescope they woulave been invisible for more than two hours. The effect of this instrument is to show an oba distance of say fifty miles, as if it were but five miles."

he remarkable properties of the telescope at once commanded universal attention amongtellectual men. Galileo received applications from several quarters for his new instrument,hich it would seem that he manufactured a large number to be distributed as gifts to varioustrious personages.




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ut it was reserved for Galileo himself to make that application of the instrument to theelestial bodies by which its peculiar powers were to inaugurate the new era in astronomy.he first discovery that was made in this direction appears to have been connected with theumber of the stars. Galileo saw to his amazement that through his little tube he could coun times as many stars in the sky as his unaided eye could detect. Here was, indeed, arprise. We are now so familiar with the elementary facts of astronomy that it is not alway

asy to realise how the heavens were interpreted by the observers in those ages prior to thvention of the telescope. We can hardly, indeed, suppose that Galileo, like the majority ofose who ever thought of such matters, entertained the erroneous belief that the stars we

n the surface of a sphere at equal distances from the observer. No one would be likely toave retained his belief in such a doctrine when he saw how the number of visible stars coue increased tenfold by means of Galileo's telescope. It would have been almost impossiblefuse to draw the inference that the stars thus brought into view were still more remote

bjects which the telescope was able to reveal, just in the same way as it showed certainips to the astonished Venetians, when at the time these ships were beyond the reach of

naided vision.

alileo's celestial discoveries now succeeded each other rapidly. That beautiful Milky Way,hich has for ages been the object of admiration to all lovers of nature, never disclosed itsue nature to the eye of man till the astronomer of Padua turned on it his magic tube. Theplendid zone of silvery light was then displayed as star-dust scattered over the black ackground of the sky. It was observed that though the individual stars were too small to ben severally without optical aid, yet such was their incredible number that the celestialdiance produced that luminosity with which every stargazer was so familiar.

ut the greatest discovery made by the telescope in these early days, perhaps, indeed, theeatest discovery that the telescope has ever accomplished, was the detection of the systefour satellites revolving around the great planet Jupiter. This phenomenon was so wholly

nexpected by Galileo that, at first, he could hardly believe his eyes. However, the reality oe existence of a system of four moons attending the great planet was soon established

eyond all question. Numbers of great personages crowded to Galileo to see for themselvesis beautiful miniature representing the sun with its system of revolving planets.

f course there were, as usual, a few incredulous people who refused to believe the assertat four more moving bodies had to be added to the planetary system. They scoffed at the

otion; they said the satellites may have been in the telescope, but that they were not in thky. One sceptical philosopher is reported to have affirmed, that even if he saw the moons piter himself he would not believe in them, as their existence was contrary to the principlcommon-sense!

here can be no doubt that a special significance attached to the new discovery at this




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articular epoch in the history of science. It must be remembered that in those days theoctrine of Copernicus, declaring that the sun, and not the earth, was the centre of thestem, that the earth revolved on its axis once a day, and that it described a mighty circleund the sun once a year, had only recently been promulgated. This new view of the schenature had been encountered with the most furious opposition. It may possibly have beeat Galileo himself had not felt quite confident in the soundness of the Copernican theory,ior to the discovery of the satellites of Jupiter. But when a picture was there exhibited in

hich a number of relatively small globes were shown to be revolving around a single largeobe in the centre, it seemed impossible not to feel that the beautiful spectacle so displayeas an emblem of the relations of the planets to the sun. It was thus made manifest to Gaat the Copernican theory of the planetary system must be the true one. The momentous

mport of this opinion upon the future welfare of the great philosopher will presently appea

would seem that Galileo regarded his residence at Padua as a state of undesirable exileom his beloved Tuscany. He had always a yearning to go back to his own country and at e desired opportunity presented itself. For now that Galileo's fame had become so great,

rand Duke of Tuscany desired to have the philosopher resident at Florence, in the belief the would shed lustre on the Duke's dominions. Overtures were accordingly made to Galileond the consequence was that in 1616 we find him residing at Florence, bearing the title ofathematician and Philosopher to the Grand Duke.

wo daughters, Polissena and Virginia, and one son, Vincenzo, had been born to Galileo inadua. It was the custom in those days that as soon as the daughter of an Italian gentlemaad grown up, her future career was somewhat summarily decided. Either a husband was te forthwith sought out, or she was to enter the convent with the object of taking the veil a

ofessed nun. It was arranged that the two daughters of Galileo, while still scarcely morean children, should both enter the Franciscan convent of St. Matthew, at Arcetri. The elde

aughter Polissena, took the name of Sister Maria Celeste, while Virginia became Sisterrcangela. The latter seems to have been always delicate and subject to prolongedelancholy, and she is of but little account in the narrative of the life of Galileo. But Sisteraria Celeste, though never leaving the convent, managed to preserve a close intimacy witer beloved father. This was maintained only partly by Galileo's visits, which were veryegular and were, indeed, often suspended for long intervals. But his letters to this daughere evidently frequent and affectionate, especially in the latter part of his life. Most

nfortunately, however, all his letters have been lost. There are grounds for believing thatey were deliberately destroyed when Galileo was seized by the Inquisition, lest they shou

ave been used as evidence against him, or lest they should have compromised the convenhere they were received. But Sister Maria Celeste's letters to her father have happily beeneserved, and most touching these letters are. We can hardly read them without thinking

ow the sweet and gentle nun would have shrunk from the idea of their publication.

er loving little notes to her "dearest lord and father," as she used affectionately to callalileo, were almost invariably accompanied by some gift, trifling it may be, but always the




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est the poor nun had to bestow. The tender grace of these endearing communications wa the more precious to him from the fact that the rest of Galileo's relatives were of quite aorthless description. He always acknowledged the ties of his kindred in the most generousay, but their follies and their vices, their selfishness and their importunities, were ancessant source of annoyance to him, almost to the last day of his life.

n 19th December, 1625, Sister Maria Celeste writes:--

send two baked pears for these days of vigil. But as the greatest treat of all, I send you ase, which ought to please you extremely, seeing what a rarity it is at this season; and wite rose you must accept its thorns, which represent the bitter passion of our Lord, whilst teen leaves represent the hope we may entertain that through the same sacred passion w

aving passed through the darkness of the short winter of our mortal life, may attain to theightness and felicity of an eternal spring in heaven."

hen the wife and children of Galileo's shiftless brother came to take up their abode in the

hilosopher's home, Sister Maria Celeste feels glad to think that her father has now some oho, however imperfectly may fulfil the duty of looking after him. A graceful note onhristmas Eve accompanies her little gifts. She hopes that--

n these holy days the peace of God may rest on him and all the house. The largest collarnd sleeves I mean for Albertino, the other two for the two younger boys, the little dog foraby, and the cakes for everybody, except the spice-cakes, which are for you. Accept theood-will which would readily do much more."

he extraordinary forbearance with which Galileo continually placed his time, his purse, ands influence at the service of those who had repeatedly proved themselves utterly unworthhis countenance, is thus commented on by the good nun.--

Now it seems to me, dearest lord and father, that your lordship is walking in the right pathnce you take hold of every occasion that presents itself to shower continual benefits on thho only repay you with ingratitude. This is an action which is all the more virtuous anderfect as it is the more difficult."

hen the plague was raging in the neighbourhood, the loving daughter's solicitude is thusown:--

send you two pots of electuary as a preventive against the plague. The one without thebel consists of dried figs, walnuts, rue, and salt, mixed together with honey. A piece of thze of a walnut to be taken in the morning, fasting, with a little Greek wine."

he plague increasing still more, Sister Maria Celeste obtained with much difficulty, a small




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uantity of a renowned liqueur, made by Abbess Ursula, an exceptionally saintly nun. This snds to her father with the words:--

pray your lordship to have faith in this remedy. For if you have so much faith in my pooriserable prayers, much more may you have in those of such a holy person; indeed, througer merits you may feel sure of escaping all danger from the plague."

hether Galileo took the remedy we do not know, but at all events he escaped the plague.

om Galileo's new home in Florence the telescope was again directed to the skies, and agad astounding discoveries reward the atronomer's labours. The great success which he hadet with in studying Jupiter naturally led Galileo to look at Saturn. Here he saw a spectaclehich was sufficiently amazing, though he failed to interpret it accurately. It was quiteanifest that Saturn did not exhibit a simple circular disc like Jupiter, or like Mars. It seemeGalileo as if the planet consisted of three bodies, a large globe in the centre, and a small

ne on each side. The enigmatical nature of the discovery led Galileo to announce it in an

nigmatical manner. He published a string of letters which, when duly transposed, made upntence which affirmed that the planet Saturn was threefold. Of course we now know thatis remarkable appearance of the planet was due to the two projecting portions of the ringith the feeble power of Galileo's telescope, these seemed merely like small globes or

ppendages to the large central body.

he last Of Galileo's great astronomical discoveries related to the libration of the moon. I that the detection of this phenomenon shows his acuteness of observation more remarkablyan does any one of his other achievements with the telescope. It is well known that the

oon constantly keeps the same face turned towards the earth. When, however, carefuleasurements have been made with regard to the spots and marks on the lunar surface, itund that there is a slight periodic variation which permits us to see now a little to the easto the west, now a little to the north or to the south of the average lunar disc.

ut the circumstances which make the career of Galileo so especially interesting from theographer's point of view, are hardly so much the triumphs that he won as the sufferings te endured. The sufferings and the triumphs were, however, closely connected, and it isting that we should give due consideration to what was perhaps the greatest drama in th

story of science.

n the appearance of the immortal work of Copernicus, in which it was taught that the earttated on its axis, and that the earth, like the other planets, revolved round the sun,thodoxy stood aghast. The Holy Roman Church submitted this treatise, which bore the na

De Revolutionibus Orbium Coelestium," to the Congregation of the Index. After duexamination it was condemned as heretical in 1615. Galileo was suspected, on no doubtxcellent grounds, of entertaining the objectionable views of Copernicus. He was according




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ivately summoned before Cardinal Bellarmine on 26th February 1616, and duly admonishat he was on no account to teach or to defend the obnoxious doctrines. Galileo was muchstressed by this intimation. He felt it a serious matter to be deprived of the privilege of scoursing with his friends about the Copernican system, and of instructing his disciples in inciples of the great theory of whose truth he was perfectly convinced. It pained him,

owever, still more to think, devout Catholic as he was, that such suspicions of his ferventegiance to his Church should ever have existed, as were implied by the words and moniti

Cardinal Bellarmine.

1616, Galileo had an interview with Pope Paul V., who received the great astronomer veaciously, and walked up and down with him in conversation for three-quarters of an houralileo complained to his Holiness of the attempts made by his enemies to embarrass him we authorities of the Church, but the Pope bade him be comforted. His Holiness had himse

o doubts of Galileo's orthodoxy, and he assured him that the Congregation of the Indexould give Galileo no further trouble so long as Paul V. was in the chair of St. Peter.

n the death of Paul V. in 1623, Maffeo Barberini was elected Pope, as Urban VIII. This newope, while a cardinal, had been an intimate friend of Galileo's, and had indeed written Laterses in praise of the great astronomer and his discoveries. It was therefore not unnaturalalileo to think that the time had arrived when, with the use of due circumspection, he migontinue his studies and his writings, without fear of incurring the displeasure of the Churchdeed, in 1624, one of Galileo's friends writing from Rome, urges Galileo to visit the city

gain, and added that--

Under the auspices of this most excellent, learned, and benignant Pontiff, science must

ourish. Your arrival will be welcome to his Holiness. He asked me if you were coming, andhen, and in short, he seems to love and esteem you more than ever."

he visit was duly paid, and when Galileo returned to Florence, the Pope wrote a letter fromhich the following is an extract, commanding the philosopher to the good offices of theoung Ferdinand, who had shortly before succeeded his father in the Grand Duchy of Tusca

We find in Galileo not only literary distinction, but also the love of piety, and he is also strothose qualities by which the pontifical good-will is easily obtained. And now, when he has

een brought to this city to congratulate us on our elevation, we have very lovingly embracm; nor can we suffer him to return to the country whither your liberality calls him, withoun ample provision of pontifical love. And that you may know how dear he is to us, we havelled to give him this honourable testimonial of virtue and piety. And we further signify tha

very benefit which you shall confer upon him, imitating or even surpassing your father'sberality, will conduce to our gratification."

he favourable reception which had been accorded to him by Pope Urban VIII. seems to ha




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d Galileo to expect that there might be some corresponding change in the attitude of theapal authorities on the great question of the stability of the earth. He accordingly proceedeth the preparation of the chief work of his life, "The Dialogue of the two Systems." It wasbmitted for inspection by the constituted authorities. The Pope himself thought that, if a

onditions which he laid down were duly complied with, there could be no objection to theublication of the work. In the first place, the title of the book was to be so carefully worde to show plainly that the Copernican doctrine was merely to be regarded as an hypothesi

nd not as a scientific fact. Galileo was also instructed to conclude the book with specialguments which had been supplied by the Pope himself, and which appeared to his Holinebe quite conclusive against the new doctrine of Copernicus.

ormal leave for the publication of the Dialogue was then given to Galileo by the Inquisitoreneral, and it was accordingly sent to the press. It might be thought that the anxieties of tronomer about his book would then have terminated. As a matter of fact, they had not yriously begun. Riccardi, the Master of the Sacred Palace, having suddenly had some furthisgivings, sent to Galileo for the manuscript while the work was at the printer's, in order t

e doctrine it implied might be once again examined. Apparently, Riccardi had come to theonclusion that he had not given the matter sufficient attention, when the authority to go toess had been first and, perhaps, hastily given. Considerable delay in the issue of the bookas the result of these further deliberations. At last, however, in June, 1632, Galileo's greatork, "The Dialogue of the two Systems," was produced for the instruction of the world,ough the occasion was fraught with ruin to the immortal author.

he book, on its publication, was received and read with the greatest avidity. But presentlyaster of The Sacred Palace found reason to regret that he had given his consent to its

ppearance. He accordingly issued a peremptory order to sequestrate every copy in Italy. Tdden change in the Papal attitude towards Galileo formed the subject of a strongmonstrance addressed to the Roman authorities by the Grand Duke of Tuscany. The Popemself seemed to have become impressed all at once with the belief that the work containatter of an heretical description. The general interpretation put upon the book seems toave shown the authorities that they had mistaken its true tendency, notwithstanding the fat it had been examined again and again by theologians deputed for the duty. To the

ommunication from the Grand Duke the Pope returned answer, that he had decided tobmit the book to a congregation of "learned, grave, and saintly men," who would weigh

very word in it. The views of his Holiness personally on the subject were expressed in hiselief that the Dialogue contained the most perverse matter that could come into a reader'sands.

he Master of the Sacred Palace was greatly blamed by the authorities for having given hisnction to its issue. He pleaded that the book had not been printed in the precise terms ofe original manuscript which had been submitted to him. It was also alleged that Galileo h

ot adhered to his promise of inserting properly the arguments which the Pope himself hadven in support of the old and orthodox view. One of these had, no doubt, been introduced




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ut, so far from mending Galileo's case, it had made matters really look worse for the poorhilosopher. The Pope's argument had been put into the mouth of one of the characters in alogue named "Simplicio." Galileo's enemies maintained that by adopting such a method e expression of his Holiness's opinion, Galileo had intended to hold the Pope himself up todicule. Galileo's friends maintained that nothing could have been farther from his intentionems, however, highly probable that the suspicions thus aroused had something to say to dden change of front on the part of the Papal authorities.

n 1st October, 1632, Galileo received an order to appear before the Inquisition at Rome oe grave charge of heresy. Galileo, of course, expressed his submission, but pleaded for aspite from compliance with the summons, on the ground of his advanced age and his fail

ealth. The Pope was, however, inexorable; he said that he had warned Galileo of his danghile he was still his friend. The command could not be disobeyed. Galileo might perform turney as slowly as he pleased, but it was imperatively necessary for him to set forth and nce.

n 20th January, 1633, Galileo started on his weary journey to Rome, in compliance with theremptory summons. On 13th February he was received as the guest of Niccolini, the Tusmbassador, who had acted as his wise and ever-kind friend throughout the whole affair. Itemed plain that the Holy Office were inclined to treat Galileo with as much clemency and

onsideration as was consistent with the determination that the case against him should beoceeded with to the end. The Pope intimated that in consequence of his respect for therand Duke of Tuscany he should permit Galileo to enjoy the privilege, quite unprecedenter a prisoner charged with heresy, of remaining as an inmate in the ambassador's house. H

ught, strictly, to have been placed in the dungeons of the Inquisition. When the examinati

the accused had actually commenced, Galileo was confined, not, indeed, in the dungeonut in comfortable rooms at the Holy Office.

y the judicious and conciliatory language of submission which Niccolini had urged Galileo tse before the Inquisitors, they were so far satisfied that they interceded with the Pope for lease. During the remainder of the trial Galileo was accordingly permitted to go back to th

mbassador's, where he was most heartily welcomed. Sister Maria Celeste, evidently thinkinis meant that the whole case was at an end, thus expresses herself:--

The joy that your last dear letter brought me, and the having to read it over and over to thuns, who made quite a jubilee on hearing its contents, put me into such an excited state tlast I got a severe attack of headache."

his defence Galileo urged that he had already been acquitted in 1616 by Cardinalellarmine, when a charge of heresy was brought against him, and he contended thatnything he might now have done, was no more than he had done on the preceding occasihen the orthodoxy of his doctrines received solemn confirmation. The Inquisition seemed




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ertainly inclined to clemency, but the Pope was not satisfied. Galileo was accordinglymmoned again on the 21st June. He was to be threatened with torture if he did notrthwith give satisfactory explanations as to the reasons which led him to write the Dialoguthis proceeding the Pope assured the Tuscan ambassador that he was treating Galileo w

e utmost consideration possible in consequence of his esteem and regard for the Granduke, whose servant Galileo was. It was, however, necessary that some exemplaryunishment be meted out to the astronomer, inasmuch as by the publication of the Dialogu

e had distinctly disobeyed the injunction of silence laid upon him by the decree of 1616. Nas it admissible for Galileo to plead that his book had been sanctioned by the Master of thacred College, to whose inspection it had been again and again submitted. It was held, the Master of the Sacred College had been unaware of the solemn warning the philosopher

ad already received sixteen years previously, it was the duty of Galileo to have drawn histention to that fact.

n the 22nd June, 1633, Galileo was led to the great hall of the Inquisition, and compelled neel before the cardinals there assembled and hear his sentence. In along document, mos

aborately drawn up, it is definitely charged against Galileo that, in publishing the Dialoguee committed the essentially grave error of treating the doctrine of the earth's motion as opdiscussion. Galileo knew, so the document affirmed, that the Church had emphaticallyonounced this notion to be contrary to Holy Writ, and that for him to consider a doctrine gmatized as having any shadow of probability in its favour was an act of disrespect to the

uthority of the Church which could not be overlooked. It was also charged against Galileoat in his Dialogue he has put the strongest arguments into the mouth, not of those whopported the orthodox doctrine, but of those who held the theory as to the earth's motionhich the Church had so deliberately condemned.

ter due consideration of the defence made by the prisoner, it was thereupon decreed thae had rendered himself vehemently suspected of heresy by the Holy Office, and inonsequence had incurred all the censures and penalties of the sacred canons, and otherecrees promulgated against such persons. The graver portion of these punishments wouldmitted, if Galileo would solemnly repudiate the heresies referred to by an abjuration to beonounced by him in the terms laid down.

the same time it was necessary to mark, in some emphatic manner, the serious offence

hich had been committed, so that it might serve both as a punishment to Galileo and as aarning to others. It was accordingly decreed that he should be condemned to imprisonmethe Holy Office during the pleasure of the Papal authorities, and that he should recite oncweek for three years the seven Penitential Psalms.

hen followed that ever-memorable scene in the great hall of the Inquisition, in which theged and infirm Galileo, the inventor of the telescope and the famous astronomer, knelt do

abjure before the most eminent and reverend Lords Cardinal, Inquisitors Generalroughout the Christian Republic against heretical depravity. With his hands on the Gospel




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alileo was made to curse and detest the false opinion that the sun was the centre of theniverse and immovable, and that the earth was not the centre of the same, and that itoved. He swore that for the future he will never say nor write such things as may bring hnder suspicion, and that if he does so he submits to all the pains and penalties of the sacrnons. This abjuration was subsequently read in Florence before Galileo's disciples, who h

een specially summoned to attend.

has been noted that neither on the first occasion, in 1616, nor on the second in 1633, dide reigning Pope sign the decrees concerning Galileo. The contention has accordingly beenade that Paul V. and Urban VIII. are both alike vindicated from any technical responsibilitr the attitude of the Romish Church towards the Copernican doctrines. The significance ofis circumstance has been commented on in connection with the doctrine of the infallibilitye Pope.

e can judge of the anxiety felt by Sister Maria Celeste about her beloved father during therrible trials. The wife of the ambassador Niccolini, Galileo's steadfast friend, most kindly

rote to give the nun whatever quieting assurances the case would permit. There is anewed flow of these touching epistles from the daughter to her father. Thus she sendsord--

The news of your fresh trouble has pierced my soul with grief all the more that it came qunexpectedly."

nd again, on hearing that he had been permitted to leave Rome, she writes--

wish I could describe the rejoicing of all the mothers and sisters on hearing of your happrival at Siena. It was indeed most extraordinary. On hearing the news the Mother Abbess

nd many of the nuns ran to me, embracing me and weeping for joy and tenderness."

he sentence of imprisonment was at first interpreted leniently by the Pope. Galileo wasowed to reside in qualified durance in the archbishop's house at Siena. Evidently theeatest pain that he endured arose from the forced separation from that daughter, whom

ad at last learned to love with an affection almost comparable with that she bore to him. Sad often told him that she never had any pleasure equal to that with which she rendered arvice to her father. To her joy, she discovers that she can relieve him from the task of citing the seven Penitential Psalms which had been imposed as a Penance:--

began to do this a while ago," she writes, "and it gives me much pleasure. First, becausem persuaded that prayer in obedience to Holy Church must be efficacious; secondly, in ord

save you the trouble of remembering it. If I had been able to do more, most willingly wohave entered a straiter prison than the one I live in now, if by so doing I could have set yliberty."




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ster Maria Celeste was gradually failing in health, but the great privilege was accorded to being able once again to embrace her beloved lord and master. Galileo had, in fact, been

ermitted to return to his old home; but on the very day when he heard of his daughter'seath came the final decree directing him to remain in his own house in perpetual solitude.

mid the advancing infirmities of age, the isolation from friends, and the loss of his daughte

alileo once again sought consolation in hard work. He commenced his famous dialogue onotion. Gradually, however, his sight began to fail, and blindness was at last added to hisher troubles. On January 2nd, 1638, he writes to Diodati:--

Alas, your dear friend and servant, Galileo, has been for the last month perfectly blind, soat this heaven, this earth, this universe which I by my marvellous discoveries and clear

emonstrations have enlarged a hundred thousand times beyond the belief of the wise menygone ages, henceforward is for me shrunk into such a small space as is filled by my ownodily sensations."

ut the end was approaching--the great philosopher, was attacked by low fever, from whice died on the 8th January, 1643.

KEPLER.

hile the illustrious astronomer, Tycho Brahe, lay on his death-bed, he had an interviewhich must ever rank as one of the important incidents in the history of science. The life ofycho had been passed, as we have seen, in the accumulation of vast stores of careful

bservations of the positions of the heavenly bodies. It was not given to him to deduce froms splendid work the results to which they were destined to lead. It was reserved for anothtronomer to distil, so to speak, from the volumes in which Tycho's figures were recorded,e great truths of the universe which those figures contained. Tycho felt that his work quired an interpreter, and he recognised in the genius of a young man with whom he wa

cquainted the agent by whom the world was to be taught some of the great truths of natuo the bedside of the great Danish astronomer the youthful philosopher was summoned, anth his last breath Tycho besought of him to spare no labour in the performance of thoselculations, by which alone the secrets of the movements of the heavens could be revealed

he solemn trust thus imposed was duly accepted, and the man who accepted it bore themmortal name of Kepler.

epler was born on the 27th December, 1571, at Weil, in the Duchy of Wurtemberg. It wouem that the circumstances of his childhood must have been singularly unhappy. His fathe

prung from a well-connected family, was but a shiftless and idle adventurer; nor was theeat astronomer much more fortunate in his other parent. His mother was an ignorant andmpered woman; indeed, the ill-assorted union came to an abrupt end through the desertthe wife by her husband when their eldest son John, the hero of our present sketch, was




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ghteen years old. The childhood of this lad, destined for such fame, was still furthermbittered by the circumstance that when he was four years old he had a severe attack of mall-pox. Not only was his eyesight permanently injured, but even his constitution appearsave been much weakened by this terrible malady.

seems, however, that the bodily infirmities of young John Kepler were the immediate cauhis attention being directed to the pursuit of knowledge. Had the boy been fitted like oth

oys for ordinary manual work, there can be hardly any doubt that to manual work his lifeust have been devoted. But, though his body was feeble, he soon gave indications of theossession of considerable mental power. It was accordingly thought that a suitable spheres talents might be found in the Church which, in those days, was almost the only professiat afforded an opening for an intellectual career. We thus find that by the time John Keplas seventeen years old he had attained a sufficient standard of knowledge to entitle him tdmission on the foundation of the University at Tubingen.

the course of his studies at this institution he seems to have divided his attention equally

etween astronomy and divinity. It not unfrequently happens that when a man has attainedonsiderable proficiency in two branches of knowledge he is not able to see very clearly inhich of the two pursuits his true vocation lies. His friends and onlookers are often able todge more wisely than he himself can do as to which Of the two lines it would be better fom to pursue. This incapacity for perceiving the path in which greatness awaited him, existthe case of Kepler. Personally, he inclined to enter the ministry, in which a promising caremed open to him. He yielded, however, to friends, who evidently knew him better than

new himself, and accepted in 1594, the important Professorship of astronomy which hadeen offered to him in the University of Gratz.

is difficult for us in these modern days to realise the somewhat extraordinary duties whicere expected from an astronomical professor in the sixteenth century. He was, of course,quired to employ his knowledge of the heavens in the prediction of eclipses, and of theovements of the heavenly bodies generally. This seems reasonable enough; but what we ot prepared to accept is the obligation which lay on the astronomers to predict the fates oations and the destinies of individuals.

must be remembered that it was the almost universal belief in those days, that all the

elestial spheres revolved in some mysterious fashion around the earth, which appeared bye most important body in the universe. It was imagined that the sun, the moon, and thears indicated, in the vicissitudes of their movements, the careers of nations and of dividuals. Such being the generally accepted notion, it seemed to follow that a professor was charged with the duty of expounding the movements of the heavenly bodies mustecessarily be looked to for the purpose of deciphering the celestial decrees regarding the fman which the heavenly luminaries were designed to announce.




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epler threw himself with characteristic ardour into even this fantastic phase of the labourse astronomical professor; he diligently studied the rules of astrology, which the fancies of

ntiquity had compiled. Believing sincerely as he did in the connection between the aspect e stars and the state of human affairs, he even thought that he perceived, in the events os own life, a corroboration of the doctrine which affirmed the influence of the planets upoe fate of individuals.

ut quite independently of astrology there seem to have been many other delusions currenmong the philosophers of Kepler's time. It is now almost incomprehensible how the ablesten of a few centuries ago should have entertained such preposterous notions, as they didth respect to the system of the universe. As an instance of what is here referred to, we mte the extraordinary notion which, under the designation of a discovery, first brought Kepto fame. Geometers had long known that there were five, but no more than five, regular

olid figures. There is, for instance, the cube with six sides, which is, of course, the mostmiliar of these solids. Besides the cube there are other figures of four, eight, twelve, and

wenty sides respectively. It also happened that there were five planets, but no more than

ve, known to the ancients, namely, Mercury, Venus, Mars, Jupiter, and Saturn. To Kepler'svely imaginations this coincidence suggested the idea that the five regular solidsorresponded to the five planets, and a number of fancied numerical relations were adducen the subject. The absurdity of this doctrine is obvious enough, especially when we observat, as is now well known, there are two large planets, and a host of small planets, over a

bove the magical number of the regular solids. In Kepler's time, however, this doctrine wao far from being regarded as absurd, that its announcement was hailed as a great intellectumph. Kepler was at once regarded with favour. It seems, indeed, to have been thercumstance which brought him into correspondence with Tycho Brahe. By its means also

ecame known to Galileo.

he career of a scientific professor in those early days appears generally to have been marky rather more striking vicissitudes than usually befall a professor in a modern university.epler was a Protestant, and as such he had been appointed to his professorship at Gratz. Ahange, however, having taken place in the religious belief entertained by the ruling powere University, the Protestant professors were expelled. It seems that special influence havi

een exerted in Kepler's case on account of his exceptional eminence, he was recalled to Gnd reinstated in the tenure of his chair. But his pupils had vanished, so that the great

tronomer was glad to accept a post offered him by Tycho Brahe in the observatory whiche latter had recently established near Prague.

n Tycho's death, which occurred soon after, an opening presented itself which gave Keplee opportunity his genius demanded. He was appointed to succeed Tycho in the position o

mperial mathematician. But a far more important point, both for Kepler and for science, waat to him was confided the use of Tycho's observations. It was, indeed, by the discussion

ycho's results that Kepler was enabled to make the discoveries which form such an importart of astronomical history.




Great Astronomers

epler must also be remembered as one of the first great astronomers who ever had theivilege of viewing celestial bodies through a telescope. It was in 1610 that he first held in

ands one of those little instruments which had been so recently applied to the heavens byalileo. It should, however, be borne in mind that the epoch-making achievements of Kepled not arise from any telescopic observations that he made, or, indeed, that any one elseade. They were all elaborately deduced from Tycho's measurements of the positions of th

anets, obtained with his great instruments, which were unprovided with telescopicsistance.

o realise the tremendous advance which science received from Kepler's great work, it is tonderstood that all the astronomers who laboured before him at the difficult subject of theelestial motions, took it for granted that the planets must revolve in circles. If it did notppear that a planet moved in a fixed circle, then the ready answer was provided by Ptolemeory that the circle in which the planet did move was itself in motion, so that its centre

escribed another circle.

hen Kepler had before him that wonderful series of observations of the planet, Mars, whicad been accumulated by the extraordinary skill of Tycho, he proved, after much labour, the movements of the planet refused to be represented in a circular form. Nor would it do tppose that Mars revolved in one circle, the centre of which revolved in another circle. Onch supposition could the movements of the planets be made to tally with those which Tyc

ad actually observed. This led to the astonishing discovery of the true form of a planet'sbit. For the first time in the history of astronomy the principle was laid down that theovement of a planet could not be represented by a circle, nor even by combinations of

rcles, but that it could be represented by an elliptic path. In this path the sun is situated ane of those two points in the ellipse which are known as its foci.

ery simple apparatus is needed for the drawing of one those ellipses which Kepler has shopossess such astonishing astronomical significance. Two pins are stuck through a sheet o

aper on a board, the point of a pencil is inserted in a loop of string which passes over thens, and as the pencil is moved round in such a way as to keep the string stretched, thateautiful curve known as the ellipse is delineated, while the positions of the pins indicate thwo foci of the curve. If the length of the loop of string is unchanged then the nearer the p

e together, the greater will be the resemblance between the ellipse and the circle, whereae more the pins are separated the more elongated does the ellipse become. The orbit of eat planet is, in general, one of those ellipses which approaches a nearly circular form. Itrtunately happens, however, that the orbit of Mars makes a wider departure from thercular form than any of the other important planets. It is, doubtless, to this circumstanceat we must attribute the astonishing success of Kepler in detecting the true shape of aanetary orbit. Tycho's observations would not have been sufficiently accurate to havexhibited the elliptic nature of a planetary orbit which, like that of Venus, differed very littleom a circle.




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he more we ponder on this memorable achievement the more striking will it appear. It mue remembered that in these days we know of the physical necessity which requires that aanet shall revolve in an ellipse and not in any other curve. But Kepler had no suchnowledge. Even to the last hour of his life he remained in ignorance of the existence of anatural cause which ordained that planets should follow those particular curves whicheometers know so well. Kepler's assignment of the ellipse as the true form of the planetar

bit is to be regarded as a brilliant guess, the truth of which Tycho's observations enabledm to verify. Kepler also succeeded in pointing out the law according to which the velocity planet at different points of its path could be accurately specified. Here, again, we have todmire the sagacity with which this marvellously acute astronomer guessed the deep truth ature. In this case also he was quite unprovided with any reason for expecting from physiinciples that such a law as he discovered must be obeyed. It is quite true that Kepler had

ome slight knowledge of the existence of what we now know as gravitation. He had evennunciated the remarkable doctrine that the ebb and flow of the tide must be attributed to traction of the moon on the waters of the earth. He does not, however, appear to have h

ny anticipation of those wonderful discoveries which Newton was destined to make a littleter, in which he demonstrated that the laws detected by Kepler's marvellous acumen wereecessary consequences of the principle of universal gravitation.

o appreciate the relations of Kepler and Tycho it is necessary to note the very different wawhich these illustrious astronomers viewed the system of the heavens. It should be

bserved that Copernicus had already expounded the true system, which located the sun ate centre of the planetary system. But in the days of Tycho Brahe this doctrine had not as

ommanded universal assent. In fact, the great observer himself did not accept the new vie

Copernicus. It appeared to Tycho that the earth not only appeared to be the centre of ings celestial, but that it actually was the centre. It is, indeed, not a little remarkable thatudent of the heavens so accurate as Tycho should have deliberately rejected the Copernicoctrine in favour of the system which now seems so preposterous. Throughout his greatreer, Tycho steadily observed the places of the sun, the moon, and the planets, and aseadily maintained that all those bodies revolved around the earth fixed in the centre. Kepowever, had the advantage of belonging to the new school. He utilised the observations oycho in developing the great Copernican theory whose teaching Tycho stoutly resisted.

erhaps a chapter in modern science may illustrate the intellectual relation of these greaten. The revolution produced by Copernicus in the doctrine of the heavens has often been

kened to the revolution which the Darwinian theory produced in the views held by biologis to life on this earth. The Darwinian theory did not at first command universal assent eve

mong those naturalists whose lives had been devoted with the greatest success to the stuorganisms. Take, for instance, that great naturalist, Professor Owen, by whose labours v

xtension has been given to our knowledge of the fossil animals which dwelt on the earth inast ages. Now, though Owens researches were intimately connected with the great labourDarwin, and afforded the latter material for his epoch-making generalization, yet Owen




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eliberately refused to accept the new doctrines. Like Tycho, he kept on rigidly accumulatins facts under the influence of a set of ideas as to the origin of living forms which are nowniversally admitted to be erroneous. If, therefore, we liken Darwin to Copernicus, and OweTycho, we may liken the biologists of the present day to Kepler, who interpreted the resuaccurate observation upon sound theoretical principles.

reading the works of Kepler in the light of our modern knowledge we are often struck by

e extent to which his perception of the sublimest truths in nature was associated with theost extravagant errors and absurdities. But, of course, it must be remembered that he wran age in which even the rudiments of science, as we now understand it, were almost

ntirely unknown.

may well be doubted whether any joy experienced by mortals is more genuine than thathich rewards the successful searcher after natural truths. Every science-worker, be hisforts ever so humble, will be able to sympathise with the enthusiastic delight of Kepler whlast, after years of toil, the glorious light broke forth, and that which he considered to be

e greatest of his astonishing laws first dawned upon him. Kepler rightly judged that theumber of days which a planet required to perform its voyage round the sun must beonnected in some manner with the distance from the planet to the sun; that is to say, withe radius of the planet's orbit, inasmuch as we may for our present object regard the planbit as circular.

ere, again, in his search for the unknown law, Kepler had no accurate dynamical principlesuide his steps. Of course, we now know not only what the connection between the planet'stance and the planet's periodic time actually is, but we also know that it is a necessary

onsequence of the law of universal gravitation. Kepler, it is true, was not without certainrmises on the subject, but they were of the most fanciful description. His notions of theanets, accurate as they were in certain important respects, were mixed up with vague ide to the properties of metals and the geometrical relations of the regular solids. Above all,asoning was penetrated by the supposed astrological influences of the stars and theirgnificant relation to human fate. Under the influence of such a farrago of notions, Keplersolved to make all sorts of trials in his search for the connection between the distance of anet from the sun and the time in which the revolution of that planet was accomplished.

was quite easily demonstrated that the greater the distance of the planet from the sun thnger was the time required for its journey. It might have been thought that the time woue directly proportional to the distance. It was, however, easy to show that this suppositiond not agree with the fact. Finding that this simple relation would not do, Kepler undertookast series of calculations to find out the true method of expressing the connection. At last,ter many vain attempts, he found, to his indescribable joy, that the square of the time inhich a planet revolves around the sun was proportional to the cube of the average distancthe planet from that body.




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he extraordinary way in which Kepler's views on celestial matters were associated with theldest speculations, is well illustrated in the work in which he propounded his splendidscovery just referred to. The announcement of the law connecting the distances of theanets from the sun with their periodic times, was then mixed up with a preposterous

onception about the properties of the different planets. They were supposed to be associath some profound music of the spheres inaudible to human ears, and performed only for

enefit of that being whose soul formed the animating spirit of the sun.

epler was also the first astronomer who ever ventured to predict the occurrence of thatmarkable phenomenon, the transit of a planet in front of the sun's disc. He published, in

629, a notice to the curious in things celestial, in which he announced that both of theanets, Mercury and Venus, were to make a transit across the sun on specified days in thenter of 1631. The transit of Mercury was duly observed by Gassendi, and the transit of

enus also took place, though, as we now know, the circumstances were such that it was nossible for the phenomenon to be witnessed by any European astronomer.

addition to Kepler's discoveries already mentioned, with which his name will be for eversociated, his claim on the gratitude of astronomers chiefly depends on the publication of mous Rudolphine tables. In this remarkable work means are provided for finding the placthe planets with far greater accuracy than had previously been attainable.

epler, it must be always remembered, was not an astronomical observer. It was his functideal with the observations made by Tycho, and, from close study and comparison of thesults, to work out the movements of the heavenly bodies. It was, in fact, Tycho whoovided as it were the raw material, while it was the genius of Kepler which wrought thataterial into a beautiful and serviceable form. For more than a century the Rudolphine tablere regarded as a standard astronomical work. In these days we are accustomed to find tovements of the heavenly bodies set forth with all desirable exactitude in the NAUTICALLMANACK, and the similar publication issued by foreign Governments. Let it be rememberat it was Kepler who first imparted the proper impulse in this direction.

hen Kepler was twenty-six he married an heiress from Styria, who, though only twenty-thears old, had already had some experience in matrimony. Her first husband had died; andas after her second husband had divorced her that she received the addresses of Kepler. ll not be surprising to hear that his domestic affairs do not appear to have been particula

appy, and his wife died in 1611. Two years later, undeterred by the want of success in hisst venture, he sought a second partner, and he evidently determined not to make a mistais time. Indeed, the methodical manner in which he made his choice of the lady to whomould propose has been duly set forth by him and preserved for our edification. With somelf-assurance he asserts that there were no fewer than eleven spinsters desirous of sharins joys and sorrows. He has carefully estimated and recorded the merits and demerits of ethese would-be brides. The result of his deliberations was that he awarded himself to an




Great Astronomers

phan girl, destitute even of a portion. Success attended his choice, and his second marriaems to have proved a much more suitable union than his first. He had five children by thst wife and seven by the second.

he years of Kepler's middle life were sorely distracted by a trouble which, though notncommon in those days, is one which we find it difficult to realise at the present time. Hisother, Catherine Kepler, had attained undesirable notoriety by the suspicion that she was

uilty of witchcraft. Years were spent in legal investigations, and it was only after unceasingxertions on the part of the astronomer for upwards of a twelvemonth that he was finally a

procure her acquittal and release from prison.

is interesting for us to note that at one time there was a proposal that Kepler should forss native country and adopt England as a home. It arose in this wise. The great man wasstressed throughout the greater part of his life by pecuniary anxieties. Finding him in a stthis description, the English ambassador in Venice, Sir Henry Wotton, in the year 1620,

esought Kepler to come over to England, where he assured him that he would obtain a

vourable reception, and where, he was able to add, Kepler's great scientific work wasready highly esteemed. But his efforts were unavailing; Kepler would not leave his ownountry. He was then forty-nine years of age, and doubtless a home in a foreign land, whereople spoke a strange tongue, had not sufficient attraction for him, even when accompanith the substantial inducements which the ambassador was able to offer. Had Kepler

ccepted this invitation, he would, in transferring his home to England, have anticipated themilar change which took place in the career of another great astronomer two centuries latwill be remembered that Herschel, in his younger days, did transfer himself to England, aus gave to England the imperishable fame of association with his triumphs.

he publication of the Rudolphine tables of the celestial movements entailed much expenseonsiderable part of this was defrayed by the Government at Venice but the balanceccasioned no little trouble and anxiety to Kepler. No doubt the authorities of those days wven less Willing to spend money on scientific matters than are the Governments of morecent times. For several years the imperial Treasury was importuned to relieve him from h

nxieties. The effects of so much worry, and of the long journeys which were involved, at laoke down Kepler's health completely. As we have already mentioned, he had never beenrong from infancy, and he finally succumbed to a fever in November, 1630, at the age of

ty-nine. He was interred at St. Peter's Church at Ratisbon.

hough Kepler had not those personal characteristics which have made his great predecessycho Brahe, such a romantic figure, yet a picturesque element in Kepler's character is notanting. It was, however, of an intellectual kind. His imagination, as well as his reasoningculties, always worked together. He was incessantly prompted by the most extraordinary

peculations. The great majority of them were in a high degree wild and chimerical, but eveow and then one of his fancies struck right to the heart of nature, and an immortal truth wought to light.




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remember visiting the observatory of one of our greatest modern astronomers, and in arge desk he showed me a multitude of photographs which he had attempted but which haot been successful, and then he showed me the few and rare pictures which had succeedend by which important truths had been revealed. With a felicity of expression which I haveten since thought of, he alluded to the contents of the desk as the "chips." They wereseless, but they were necessary incidents in the truly successful work. So it is in all great a

ood work. Even the most skilful man of science pursues many a wrong scent. Time after te goes off on some track that plays him false. The greater the man's genius and intellectusource, the more numerous will be the ventures which he makes, and the great majority ose ventures are certain to be fruitless. They are in fact, the "chips." In Kepler's case the

hips were numerous enough. They were of the most extraordinary variety and structure. Bvery now and then a sublime discovery was made of such a character as to make us regarven the most fantastic of Kepler's chips with the greatest veneration and respect.

ISAAC NEWTON.

was just a year after the death of Galileo, that an infant came into the world who washristened Isaac Newton. Even the great fame of Galileo himself must be relegated to acond place in comparison with that of the philosopher who first expounded the true theothe universe.

aac Newton was born on the 25th of December (old style), 1642, at Woolsthorpe, inncolnshire, about a half-mile from Colsterworth, and eight miles south of Grantham. Histher, Mr. Isaac Newton, had died a few months after his marriage to Harriet Ayscough, thaughter of Mr. James Ayscough, of Market Overton, in Rutlandshire. The little Isaac was ast so excessively frail and weakly that his life was despaired of. The watchful mother,

owever, tended her delicate child with such success that he seems to have thriven betteran might have been expected from the circumstances of his infancy, and he ultimately

cquired a frame strong enough to outlast the ordinary span of human life.

or three years they continued to live at Woolsthorpe, the widow's means of livelihood beinpplemented by the income from another small estate at Sewstern, in a neighbouring part

eicestershire.

1645, Mrs. Newton took as a second husband the Rev. Barnabas Smith, and on moving ter new home, about a mile from Woolsthorpe, she entrusted little Isaac to her mother, Mryscough. In due time we find that the boy was sent to the public school at Grantham, theame of the master being Stokes. For the purpose of being near his work, the embryohilosopher was boarded at the house of Mr. Clark, an apothecary at Grantham. We learnom Newton himself that at first he had a very low place in the class lists of the school, anas by no means one of those model school-boys who find favour in the eyes of the schoo




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aster by attention to Latin grammar. Isaac's first incentive to diligent study seems to haveeen derived from the circumstance that he was severely kicked by one of the boys who wabove him in the class. This indignity had the effect of stimulating young Newton's activity ch an extent that he not only attained the desired object of passing over the head of the

oy who had maltreated him, but continued to rise until he became the head of the school.

he play-hours of the great philosopher were devoted to pursuits very different from those

ost school-boys. His chief amusement was found in making mechanical toys and variousgenious contrivances. He watched day by day with great interest the workmen engaged in

onstructing a windmill in the neighbourhood of the school, the result of which was that theoy made a working model of the windmill and of its machinery, which seems to have beenuch admired, as indicating his aptitude for mechanics. We are told that Isaac also indulgesomewhat higher flights of mechanical enterprise. He constructed a carriage, the wheels

hich were to be driven by the hands of the occupant, while the first philosophical instrumee made was a clock, which was actuated by water. He also devoted much attention to theonstruction of paper kites, and his skill in this respect was highly appreciated by his

hoolfellows. Like a true philosopher, even at this stage he experimented on the bestethods of attaching the string, and on the proportions which the tail ought to have. He alade lanthorns of paper to provide himself with light as he walked to school in the dark nter mornings.

he only love affair in Newton's life appears to have commenced while he was still of tendeears. The incidents are thus described in Brewster's "Life of Newton," a work to which I amuch indebted in this chapter.

n the house where he lodged there were some female inmates, in whose company heppears to have taken much pleasure. One of these, a Miss Storey, sister to Dr. Storey, ahysician at Buckminster, near Colsterworth, was two or three years younger than Newtonnd to great personal attractions she seems to have added more than the usual allotment omale talent. The society of this young lady and her companions was always preferred to this own school-fellows, and it was one of his most agreeable occupations to construct foem little tables and cupboards, and other utensils for holding their dolls and their trinketse had lived nearly six years in the same house with Miss Storey, and there is reason toelieve that their youthful friendship gradually rose to a higher passion; but the smallness o

er portion, and the inadequacy of his own fortune, appear to have prevented theonsummation of their happiness. Miss Storey was afterwards twice married, and under theame of Mrs. Vincent, Dr. Stukeley visited her at Grantham in 1727, at the age of eighty-twnd obtained from her many particulars respecting the early history of our author. Newton'steem for her continued unabated during his life. He regularly visited her when he went toncolnshire, and never failed to relieve her from little pecuniary difficulties which seem toave beset her family."

he schoolboy at Grantham was only fourteen years of age when his mother became a wid




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r the second time. She then returned to the old family home at Woolsthorpe, bringing witer the three children of her second marriage. Her means appear to have been somewhatanty, and it was consequently thought necessary to recall Isaac from the school. Hiscently-born industry had been such that he had already made good progress in his studie

nd his mother hoped that he would now lay aside his books, and those silent meditations thich, even at this early age, he had become addicted. It was expected that, instead of sucursuits, which were deemed quite useless, the boy would enter busily into the duties of th

rm and the details of a country life. But before long it became manifest that the study of ature and the pursuit of knowledge had such a fascination for the youth that he could givetle attention to aught else. It was plain that he would make but an indifferent farmer. Heeatly preferred experimenting on his water-wheels to looking after labourers, while he fouat working at mathematics behind a hedge was much more interesting than chaffering abe price of bullocks in the market place. Fortunately for humanity his mother, like a wiseoman, determined to let her boy's genius have the scope which it required. He wasccordingly sent back to Grantham school, with the object of being trained in the knowledghich would fit him for entering the University of Cambridge.

was the 5th of June, 1660, when Isaac Newton, a youth of eighteen, was enrolled as anndergraduate of Trinity College, Cambridge. Little did those who sent him there dream thais boy was destined to be the most illustrious student who ever entered the portals of thaeat seat of learning. Little could the youth himself have foreseen that the rooms near the

ateway which he occupied would acquire a celebrity from the fact that he dwelt in them, oat the ante-chapel of his college was in good time to be adorned by that noble statue, whregarded as one of the chief art treasures of Cambridge University, both on account of itstrinsic beauty and the fact that it commemorates the fame of her most distinguished

umnus, Isaac Newton, the immortal astronomer. Indeed, his advent at the Universityemed to have been by no means auspicious or brilliant. His birth was, as we have seen,

omparatively obscure, and though he had already given indication of his capacity forflecting on philosophical matters, yet he seems to have been but ill-equipped with theutine knowledge which youths are generally expected to take with them to the Universitie

om the outset of his college career, Newton's attention seems to have been mainly directmathematics. Here he began to give evidence of that marvellous insight into the deepcrets of nature which more than a century later led so dispassionate a judge as Laplace t

onounce Newton's immortal work as pre-eminent above all the productions of the humantellect. But though Newton was one of the very greatest mathematicians that ever lived, has never a mathematician for the mere sake of mathematics. He employed his mathemati an instrument for discovering the laws of nature. His industry and genius soon brought h

nder the notice of the University authorities. It is stated in the University records that hebtained a Scholarship in 1664. Two years later we find that Newton, as well as manysidents in the University, had to leave Cambridge temporarily on account of the breaking the plague. The philosopher retired for a season to his old home at Woolsthorpe, and the

e remained until he was appointed a Fellow of Trinity College, Cambridge, in 1667. From t




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me onwards, Newton's reputation as a mathematician and as a natural philosopher steadidvanced, so that in 1669, while still but twenty-seven years of age, he was appointed to thstinguished position of Lucasian Professor of Mathematics at Cambridge. Here he found thpportunity to continue and develop that marvellous career of discovery which formed his lork.

he earliest of Newton's great achievements in natural philosophy was his detection of the

omposite character of light. That a beam of ordinary sunlight is, in fact, a mixture of a vereat number of different-coloured lights, is a doctrine now familiar to every one who has tghtest education in physical science. We must, however, remember that this discovery waally a tremendous advance in knowledge at the time when Newton announced it.

e here give the little diagram originally drawn by Newton, to explain the experiment byhich he first learned the composition of light. A sunbeam is admitted into a darkened roomrough an opening, H, in a shutter. This beam when not interfered with will travel in araight line to the screen, and there reproduce a bright spot of the same shape as the hole

e shutter. If, however, a prism of glass, A B C, be introduced so that the beam traverse iten it will be seen at once that the light is deflected from its original track. There is, howevfurther and most important change which takes place. The spot of light is not alone remoanother part of the screen, but it becomes spread out into a long band beautifully colour

nd exhibiting the hues of the rainbow. At the top are the violet rays, and then in descendider we have the indigo, blue, green, yellow, orange, and red.

he circumstance in this phenomenon which appears to have particularly arrested Newton'stention, was the elongation which the luminous spot underwent in consequence of its

assage through the prism. When the prism was absent the spot was nearly circular, but we prism was introduced the spot was about five times as long as it was broad. To ascertae explanation of this was the first problem to be solved. It seemed natural to suppose thaight be due to the thickness of the glass in the prism which the light traversed, or to the

ngle of incidence at which the light fell upon the prism. He found, however, upon carefulal, that the phenomenon could not be thus accounted for. It was not until after much

atient labour that the true explanation dawned upon him. He discovered that though theeam of white light looks so pure and so simple, yet in reality it is composed of differentlyoloured lights blended together. These are, of course, indistinguishable in the compound

eam, but they are separated or disentangled, so to speak, by the action of the prism. Theys at the blue end of the spectrum are more powerfully deflected by the action of the glaan are the rays at the red end. Thus, the rays variously coloured red, orange, yellow, greue, indigo, violet, are each conducted to a different part of the screen. In this way the prias the effect of exhibiting the constitution of the composite beam of light.

o us this now seems quite obvious, but Newton did not adopt it hastily. With characteristicution he verified the explanation by many different experiments, all of which confirmed hscovery. One of these may be mentioned. He made a hole in the screen at that part on




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hich the violet rays fell. Thus a violet ray was allowed to pass through, all the rest of theht being intercepted, and on this beam so isolated he was able to try further experiments

or instance, when he interposed another prism in its path, he found, as he expected, that as again deflected, and he measured the amount of the deflection. Again he tried the samxperiment with one of the red rays from the opposite end of the coloured band. He allowe

pass through the same aperture in the screen, and he tested the amount by which thecond prism was capable of producing deflection. He thus found, as he had expected to fin

at the second prism was more efficacious in bending the violet rays than in bending the rys. Thus he confirmed the fact that the various hues of the rainbow were each bent by aism to a different extent, violet being acted upon the most, and red the least.

ot only did Newton decompose a white beam into its constituent colours, but conversely bterposing a second prism with its angle turned upwards, he reunited the different coloursnd thus reproduced the original beam of white light. In several other ways also he illustrats famous proposition, which then seemed so startling, that white light was the result of aixture of all hues of the rainbow. By combining painters' colours in the right proportion he

d not indeed succeed in producing a mixture which would ordinarily be called white, but hbtained a grey pigment. Some of this he put on the floor of his room for comparison with ece of white paper. He allowed a beam of bright sunlight to fall upon the paper and theixed colours side by side, and a friend he called in for his opinion pronounced that underese circumstances the mixed colours looked the whiter of the two.

y repeated demonstrations Newton thus established his great discovery of the compositeharacter of light. He at once perceived that his researches had an important bearing upon inciples involved in the construction of a telescope. Those who employed the telescope fo

oking at the stars, had been long aware of the imperfections which prevented all the varioys from being conducted to the same focus. But this imperfection had hitherto beenroneously accounted for. It had been supposed that the reason why success had not beetained in the construction of a refracting telescope was due to the fact that the object glaade as it then was of a single piece, had not been properly shaped. Mathematicians had

bundantly demonstrated that a single lens, if properly figured, must conduct all rays of lighthe same focus, provided all rays experienced equal refraction in passing through the gla

ntil Newton's discovery of the composition of white light, it had been taken for granted thae several rays in a white beam were equally refrangible. No doubt if this had been the ca

perfect telescope could have been produced by properly shaping the object glass. But whewton had demonstrated that light was by no means so simple as had been supposed, itecame obvious that a satisfactory refracting telescope was an impossibility when only a sinbject lens was employed, however carefully that lens might have been wrought. Such anbjective might, no doubt, be made to conduct any one group of rays of a particular shade e same focus, but the rays of other colours in the beam of white light must necessarilyavel some-what astray. In this way Newton accounted for a great part of the difficultieshich had hitherto beset the attempts to construct a perfect refracting telescope.




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e now know how these difficulties can be, to a great extent, overcome, by employing for bjective a composite lens made of two pieces of glass possessing different qualities. To thchromatic object glasses, as they are called, the great development of astronomicalnowledge, since Newton's time, is due. But it must be remarked that, although the theoretossibility of constructing an achromatic lens was investigated by Newton, he certainly cam

the conclusion that the difficulty could not be removed by employing a composite objectith two different kinds of glass. In this his marvellous sagacity in the interpretation of natu

ems for once to have deserted him. We can, however, hardly regret that Newton failed toscover the achromatic objective, when we observe that it was in consequence of hiseeming an achromatic objective to be impossible that he was led to the invention of theflecting telescope. Finding, as he believed, that the defects of the telescope could not bemedied by any application of the principle of refraction he was led to look in quite a differrection for the improvement of the tool on which the advancement of astronomy dependehe REFRACTION of light depended as he had found, upon the colour of the light. The lawsEFLECTION were, however, quite independent of the colour. Whether rays be red or greeue or yellow, they are all reflected in precisely the same manner from a mirror. According

ewton perceived that if he could construct a telescope the action of which depended uponflection, instead of upon refraction, the difficulty which had hitherto proved an insuperabl

bstacle to the improvement of the instrument would be evaded.

or this purpose Newton fashioned a concave mirror from a mixture of copper and tin, aombination which gives a surface with almost the lustre of silver. When the light of a star pon the surface, an image of the star was produced in the focus of this mirror, and then t

mage was examined by a magnifying eye- piece. Such is the principle of the famous reflectlescope which bears the name of Newton. The little reflector which he constructed,

presented in the adjoining figure, is still preserved as one of the treasures of the Royalociety. The telescope tube had the very modest dimension of one inch in diameter. It wasowever, the precursor of a whole series of magnificent instruments, each outstripping theher in magnitude, until at last the culminating point was attained in 1845, by the

onstruction of Lord Rosse's mammoth reflector of six feet in aperture.

ewton's discovery of the composition of light led to an embittered controversy, which causo little worry to the great Philosopher. Some of those who attacked him enjoyed considerand, it must be admitted, even well-merited repute in the ranks of science. They alleged,

owever, that the elongation of the coloured band which Newton had noticed was due to ththat, or to the other--to anything, in fact, rather than to the true cause which Newtonsigned. With characteristic patience and love of truth, Newton steadily replied to each suctack. He showed most completely how utterly his adversaries had misunderstood thebject, and how slight indeed was their acquaintance with the natural phenomenon in

uestion. In reply to each point raised, he was ever able to cite fresh experiments and adduesh illustrations, until at last his opponents retired worsted from the combat.

has been often a matter for surprise that Newton, throughout his whole career, should ha




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ken so much trouble to expose the errors of those who attacked his views. He used even o this when it plainly appeared that his adversaries did not understand the subject they wescussing. A philosopher might have said, "I know I am right, and whether others think I aght or not may be a matter of concern to them, but it is certainly not a matter about whiceed trouble. If after having been told the truth they elect to remain in error, so much theorse for them; my time can be better employed than in seeking to put such people right."his, however, was not Newton's method. He spent much valuable time in overthrowing

bjections which were often of a very futile description. Indeed, he suffered a great deal ofnnoyance from the persistency, and in some cases one might almost say from the rancoure attacks which were made upon him. Unfortunately for himself, he did not possess thatpacity for sublime indifference to what men may say, which is often the happy, possessiointellects greatly inferior to his.

he subject of optics still continuing to engross Newton's attention, he followed up hissearches into the structure of the sunbeam by many other valuable investigations in

onnection with light. Every one has noticed the beautiful colours manifested in a soap-bub

ere was a subject which not unnaturally attracted the attention of one who had expoundee colours of the spectrum with such success. He perceived that similar hues were produce

y other thin plates of transparent material besides soap-bubbles, and his ingenuity wasfficient to devise a method by which the thicknesses of the different films could beeasured. We can hardly, indeed, say that a like success attended his interpretation of thehenomena to that which had been so conspicuous in his explanation of the spectrum. It

mplies no disparagement to the sublime genius of Newton to admit that the doctrines he prth as to the causes of the colours in the soap-bubbles can be no longer accepted. We mumember that Newton was a pioneer in accounting for the physical properties of light. The

cts that he established are indeed unquestionable, but the explanations which he was ledfer of some of them are seen to be untenable in the fuller light of our present knowledge

ad Newton done nothing beyond making his wonderful discoveries in light, his fame wouldave gone down to posterity as one of the greatest of Nature's interpreters. But it wasserved for him to accomplish other discoveries, which have pushed even his analysis of thnbeam into the background; it is he who has expounded the system of the universe by thscovery of the law of universal gravitation.

he age had indeed become ripe for the advent of the genius of Newton. Kepler hadscovered with marvellous penetration the laws which govern the movements of the planeound the sun, and in various directions it had been more or less vaguely felt that the

xplanation of Kepler's laws, as well as of many other phenomena, must be sought for inonnection with the attractive power of matter. But the mathematical analysis which aloneould deal with this subject was wanting; it had to be created by Newton.

Woolsthorpe, in the year 1666, Newton's attention appears to have been concentratedpon the subject of gravitation. Whatever may be the extent to which we accept the more




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ss mythical story as to how the fall of an apple first directed the attention of the philosophthe fact that gravitation must extend through space, it seems, at all events, certain that an excellent illustration of the line of reasoning which he followed. He argued in this way

he earth attracts the apple; it would do so, no matter how high might be the tree from what apple fell. It would then seem to follow that this power which resides in the earth byhich it can draw all external bodies towards it, extends far beyond the altitude of the loftieee. Indeed, we seem to find no limit to it. At the greatest elevation that has ever been

tained, the attractive power of the earth is still exerted, and though we cannot by any actxperiment reach an altitude more than a few miles above the earth, yet it is certain thatavitation would extend to elevations far greater. It is plain, thought Newton, that an applt fall from a point a hundred miles above this earth's surface, would be drawn down by thtraction, and would continually gather fresh velocity until it reached the ground. From aundred miles it was natural to think of what would happen at a thousand miles, or atundreds of thousands of miles. No doubt the intensity of the attraction becomes weaker wvery increase in the altitude, but that action would still exist to some extent, however loftyight be the elevation which had been attained.

then occurred to Newton, that though the moon is at a distance of two hundred and fortyousand miles from the earth, yet the attractive power of the earth must extend to the moe was particularly led to think of the moon in this connection, not only because the moon o much closer to the earth than are any other celestial bodies, but also because the moonn appendage to the earth, always revolving around it. The moon is certainly attracted to tarth, and yet the moon does not fall down; how is this to be accounted for? The explanatias to be found in the character of the moon's present motion. If the moon were left for aoment at rest, there can be no doubt that the attraction of the earth would begin to draw

e lunar globe in towards our globe. In the course of a few days our satellite would comeown on the earth with a most fearful crash. This catastrophe is averted by the circumstanat the moon has a movement of revolution around the earth. Newton was able to calculatom the known laws of mechanics, which he had himself been mainly instrumental inscovering, what the attractive power of the earth must be, so that the moon shall moveecisely as we find it to move. It then appeared that the very power which makes an applell at the earth's surface is the power which guides the moon in its orbit.

nce this step had been taken, the whole scheme of the universe might almost be said to

ave become unrolled before the eye of the philosopher. It was natural to suppose that juse moon was guided and controlled by the attraction of the earth, so the earth itself, in th

ourse of its great annual progress, should be guided and controlled by the supreme attracower of the sun. If this were so with regard to the earth, then it would be impossible tooubt that in the same way the movements of the planets could be explained to beonsequences of solar attraction.

was at this point that the great laws of Kepler became especially significant. Kepler hadown how each of the planets revolves in an ellipse around the sun, which is situated on o




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the foci. This discovery had been arrived at from the interpretation of observations. Keplad himself assigned no reason why the orbit of a planet should be an ellipse rather than aher of the infinite number of closed curves which might be traced around the sun. Kepler

ad also shown, and here again he was merely deducing the results from observation, thathen the movements of two planets were compared together, the squares of the periodicmes in which each planet revolved were proportional to the cubes of their mean distancesom the sun. This also Kepler merely knew to be true as a fact, he gave no demonstration

e reason why nature should have adopted this particular relation between the distance ane periodic time rather than any other. Then, too, there was the law by which Kepler with

nparalleled ingenuity, explained the way in which the velocity of a planet varies at thefferent points of its track, when he showed how the line drawn from the sun to the planetescribed equal areas around the sun in equal times. These were the materials with whichewton set to work. He proposed to infer from these the actual laws regulating the force bhich the sun guides the planets. Here it was that his sublime mathematical genius came inay. Step by step Newton advanced until he had completely accounted for all the phenome

the first place, he showed that as the planet describes equal areas in equal times about n, the attractive force which the sun exerts upon it must necessarily be directed in a stra

ne towards the sun itself. He also demonstrated the converse truth, that whatever be theature of the force which emanated from a sun, yet so long as that force was directed throe sun's centre, any body which revolved around it must describe equal areas in equal tim

nd this it must do, whatever be the actual character of the law according to which thetensity of the force varies at different parts of the planet's journey. Thus the first advanceas taken in the exposition of the scheme of the universe.

he next step was to determine the law according to which the force thus proved to reside e sun varied with the distance of the planet. Newton presently showed by a most superbfort of mathematical reasoning, that if the orbit of a planet were an ellipse and if the sunere at one of the foci of that ellipse, the intensity of the attractive force must vary inverse the square of the planet's distance. If the law had any other expression than the inverseuare of the distance, then the orbit which the planet must follow would not be an ellipse;an ellipse, it would, at all events, not have the sun in the focus. Hence he was able to shoom Kepler's laws alone that the force which guided the planets was an attractive powermanating from the sun, and that the intensity of this attractive power varied with the inve

uare of the distance between the two bodies.

hese circumstances being known, it was then easy to show that the last of Kepler's threews must necessarily follow. If a number of planets were revolving around the sun, thenpposing the materials of all these bodies were equally affected by gravitation, it can be

emonstrated that the square of the periodic time in which each planet completes its orbit oportional to the cube of the greatest diameter in that orbit.

hese superb discoveries were, however, but the starting point from which Newton entered




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series of researches, which disclosed many of the profoundest secrets in the scheme of elestial mechanics. His natural insight showed that not only large masses like the sun and arth, and the moon, attract each other, but that every particle in the universe must attractvery other particle with a force which varies inversely as the square of the distance betweeem. If, for example, the two particles were placed twice as far apart, then the intensity oe force which sought to bring them together would be reduced to one-fourth. If two

articles, originally ten miles asunder, attracted each other with a certain force, then, when

e distance was reduced to one mile, the intensity of the attraction between the two particould be increased one-hundred-fold. This fertile principle extends throughout the whole oature. In some cases, however, the calculation of its effect upon the actual problems of ature would be hardly possible, were it not for another discovery which Newton's geniusnabled him to accomplish. In the case of two globes like the earth and the moon, we musmember that we are dealing not with particles, but with two mighty masses of matter, ea

omposed of innumerable myriads of particles. Every particle in the earth does attract everyarticle in the moon with a force which varies inversely as the square of their distance. Thelculation of such attractions is rendered feasible by the following principle. Assuming that

e earth consists of materials symmetrically arranged in shells of varying densities, we mayen, in calculating its attraction, regard the whole mass of the globe as concentrated at its

entre. Similarly we may regard the moon as concentrated at the centre of its mass. In thisay the earth and the moon can both be regarded as particles in point of size, each particleaving, however, the entire mass of the corresponding globe. The attraction of one particlenother is a much more simple matter to investigate than the attraction of the myriad diffeoints of the earth upon the myriad different points of the moon.

any great discoveries now crowded in upon Newton. He first of all gave the explanation o

e tides that ebb and flow around our shores. Even in the earliest times the tides had beenown to be related to the moon. It was noticed that the tides were specially high during fuoon or during new moon, and this circumstance obviously pointed to the existence of som

onnection between the moon and these movements of the water, though as to what thatonnection was no one had any accurate conception until Newton announced the law of avitation. Newton then made it plain that the rise and fall of the water was simply a

onsequence of the attractive power which the moon exerted upon the oceans lying upon oobe. He showed also that to a certain extent the sun produces tides, and he was able toxplain how it was that when the sun and the moon both conspire, the joint result was to

oduce especially high tides, which we call "spring tides"; whereas if the solar tide was lowhile the lunar tide was high, then we had the phenomenon of "neap" tides.

ut perhaps the most signal of Newton's applications of the law of gravitation was connecteth certain irregularities in the movements of the moon. In its orbit round the earth ourtellite is, of course, mainly guided by the great attraction of our globe. If there were noher body in the universe, then the centre of the moon must necessarily perform an ellipse

nd the centre of the earth would lie in the focus of that ellipse. Nature, however, does notow the movements to possess the simplicity which this arrangement would imply, for the




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n is present as a source of disturbance. The sun attracts the moon, and the sun attracts arth, but in different degrees, and the consequence is that the moon's movement with reg

the earth is seriously affected by the influence of the sun. It is not allowed to move exacan ellipse, nor is the earth exactly in the focus. How great was Newton's achievement in

olution of this problem will be appreciated if we realise that he not only had to determineom the law of gravitation the nature of the disturbance of the moon, but he had actually tonstruct the mathematical tools by which alone such calculations could be effected.

he resources of Newton's genius seemed, however, to prove equal to almost any demandat could be made upon it. He saw that each planet must disturb the other, and in that wa

e was able to render a satisfactory account of certain phenomena which had perplexed alleceding investigators. That mysterious movement by which the pole of the earth sways

bout among the stars had been long an unsolved enigma, but Newton showed that the moasped with its attraction the protuberant mass at the equatorial regions of the earth, andus tilted the earth's axis in a way that accounted for the phenomenon which had been

nown but had never been explained for two thousand years. All these discoveries were

ought together in that immortal work, Newton's Principia."

own to the year 1687, when the "Principia" was published, Newton had lived the life of acluse at Cambridge, being entirely occupied with those transcendent researches to which

ave referred. But in that year he issued from his seclusion under circumstances of onsiderable historical interest. King James the Second attempted an invasion of the rightsnd privileges of the University of Cambridge by issuing a command that Father Francis, aenedictine monk, should be received as a Master of Arts in the University, without havingken the oaths of allegiance and supremacy. With this arbitrary command the University

ernly refused to comply. The Vice-Chancellor was accordingly summoned to answer for anct of contempt to the authority of the Crown. Newton was one of nine delegates who werehosen to defend the independence of the University before the High Court. They were ableow that Charles the Second, who had issued a MANDAMUS under somewhat similarrcumstances, had been induced after due consideration to withdraw it. This argumentppeared satisfactory, and the University gained their case. Newton's next step in public lifeas his election, by a narrow majority, as member for the University, and during the years688 and 1689, he seems to have attended to his parliamentary duties with considerablegularity.

n incident which happened in 1692 was apparently the cause of considerable disturbance ewton's equanimity, if not in his health. He had gone to early morning chapel, leaving ahted candle among his papers on his desk. Tradition asserts that his little dog "Diamond"

pset the candle; at all events, when Newton came back he found that many valuable papead perished in a conflagration. The loss of these manuscripts seems to have had a seriousfect. Indeed, it has been asserted that the distress reduced Newton to a state of mental

berration for a considerable time. This has, apparently, not been confirmed, but there is noubt that he experienced considerable disquiet, for in writing on September 13th, 1693, to




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r. Pepys, he says:

am extremely troubled at the embroilment I am in, and have neither ate nor slept well thwelvemonth, nor have my former consistency of mind."

otwithstanding the fame which Newton had achieved, by the publication of his, "Principia,nd by all his researches, the State had not as yet taken any notice whatever of the most

ustrious man of science that this or any other country has ever produced. Many of his friead exerted themselves to procure him some permanent appointment, but without successappened, however, that Mr. Montagu, who had sat with Newton in Parliament, wasppointed Chancellor of the Exchequer in 1694. Ambitious of distinction in his new office, Montagu addressed himself to the improvement of the current coin, which was then in a veebased condition. It fortunately happened that an opportunity occurred of appointing a neficial in the Mint; and Mr. Montagu on the 19th of March, 1695, wrote to offer Mr. Newtone position of warden. The salary was to be five or six hundred a year, and the businessould not require more attendance than Newton could spare. The Lucasian professor

ccepted this post, and forthwith entered upon his new duties.

he knowledge of physics which Newton had acquired by his experiments was of much useonnection with his duties at the Mint. He carried out the re-coinage with great skill in theourse of two years, and as a reward for his exertions, he was appointed, in 1697, to theastership of the Mint, with a salary between 1,200 Pounds and 1,500 Pounds per annum. 701 his duties at the Mint being so engrossing, he resigned his Lucasian professorship atambridge, and at the same time he had to surrender his fellowship at Trinity College. Thisosed his connection with the University of Cambridge. It should, however, be remarked th

a somewhat earlier stage in his career he was very nearly being appointed to an officehich might have enabled the University to retain the great philosopher within its precinctsome of his friends had almost succeeded in securing his nomination to the Provostship of ng's College, Cambridge; the appointment, however, fell through, inasmuch as the statute

ould not be evaded, which required that the Provost of King's College should be in holyders.

those days it was often the custom for illustrious mathematicians, when they hadscovered a solution for some new and striking problem, to publish that problem as a

hallenge to the world, while withholding their own solution. A famous instance of this is fowhat is known as the Brachistochrone problem, which was solved by John Bernouilli. The

ature of this problem may be mentioned. It was to find the shape of the curve along whicody would slide down from one point (A) to another point (B) in the shortest time. It mighfirst be thought that the straight line from A to B, as it is undoubtedly the shortest distan

etween the points, would also be the path of quickest descent; but this is not so. There is urved line, down which a bead, let us say, would run on a smooth wire from A to B in aorter time than the same bead would require to run down the straight wire. Bernouilli'soblem was to find out what that curve must be. Newton solved it correctly; he showed th




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e curve was a part of what is termed a cycloid--that is to say, a curve like that which isescribed by a point on the rim of a carriage-wheel as the wheel runs along the ground. Suas Newton's geometrical insight that he was able to transmit a solution of the problem one day after he had received it, to the President of the Royal Society.

1703 Newton, whose world wide fame was now established, was elected President of theoyal Society. Year after year he was re-elected to this distinguished position, and his tenu

hich lasted twenty-five years, only terminated with his life. It was in discharge of his dutie President of the Royal Society that Newton was brought into contact with Prince George enmark. In April, 1705, the Queen paid a visit to Cambridge as the guest of Dr. Bentley, ten Master of Trinity, and in a court held at Trinity Lodge on April 15th, 1705, the honour

nighthood was conferred upon the discoverer of gravitation.

rged by illustrious friends, who sought the promotion of knowledge, Newton gave histention to the publication of a new edition of the "Principia." His duties at the Mint, howev

dded to the supreme duty of carrying on his original investigations, left him but little time

e more ordinary task of the revision. He was accordingly induced to associate with himselr this purpose a distinguished young mathematician, Roger Coates, a Fellow of Trinityollege, Cambridge, who had recently been appointed Plumian Professor of Astronomy. Only 27th, 1713, Newton, by this time a favourite at Court, waited on the Queen, andesented her with a copy of the new edition of the "Principia."

hroughout his life Newton appears to have been greatly interested in theological studies, ae specially devoted his attention to the subject of prophecy. He left behind him a manuscrn the prophecies of Daniel and the Apocalypse of St. John, and he also wrote various

eological papers. Many other subjects had from time to time engaged his attention. Heudied the laws of heat; he experimented in pursuit of the dreams of the Alchymist; while hilosopher who had revealed the mechanism of the heavens found occasional relaxation inying to interpret hieroglyphics. In the last few years of his life he bore with fortitude a paiment, and on Monday, March 20th, 1727, he died in the eighty-fifth year of his age. On

uesday, March 28th, he was buried in Westminster Abbey.

hough Newton lived long enough to receive the honour that his astonishing discoveries sostly merited, and though for many years of his life his renown was much greater than tha

ny of his contemporaries, yet it is not too much to say that, in the years which have sinceapsed, Newton's fame has been ever steadily advancing, so that it never stood higher thaoes at this moment.

e hardly know whether to admire more the sublime discoveries at which he arrived, or thxtraordinary character of the intellectual processes by which those discoveries were reacheewed from either standpoint, Newton's "Principia" is incomparably the greatest work onience that has ever yet been produced.




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FLAMSTEED.

mong the manuscripts preserved at Greenwich Observatory are certain documents in whicamsteed gives an account of his own life. We may commence our sketch by quoting thellowing passage from this autobiography:--"To keep myself from idleness, and to recreateyself, I have intended here to give some account of my life, in my youth, before the actioereof, and the providences of God therein, be too far passed out of my memory; and to

bserve the accidents of all my years, and inclinations of my mind, that whosoever may lighpon these papers may see I was not so wholly taken up, either with my father's business oy mathematics, but that I both admitted and found time for other as weighty

onsiderations."

he chief interest which attaches to the name of Flamsteed arises from the fact that he wase first of the illustrious series of Astronomers Royal who have presided over Greenwichbservatory. In that capacity Flamsteed was able to render material assistance to Newton boviding him with the observations which his lunar theory required.

hn Flamsteed was born at Denby, in Derbyshire, on the 19th of August, 1646. His motheed when he was three years old, and the second wife, whom his father took three yearster, only lived until Flamsteed was eight, there being also two younger sisters. In hisoyhood the future astronomer tells us that he was very fond of those romances which affeoy's imagination, but as he writes, "At twelve years of age I left all the wild ones and betoyself to read the better sort of them, which, though they were not probable, yet carried neming impossibility in the picturing." By the time Flamsteed was fifteen years old he had

mbarked in still more serious work, for he had read Plutarch's "Lives," Tacitus' "Romanstory," and many other books of a similar description. In 1661 he became ill with somerious rheumatic affection, which obliged him to be withdrawn from school. It was then foe first time that he received the rudiments of a scientific education. He had, however,tained his sixteenth year before he made any progress in arithmetic. He tells us how histher taught him "the doctrine of fractions," and "the golden rule of three"--lessons which emed to have learned easily and quickly. One of the books which he read at this timerected his attention to astronomical instruments, and he was thus led to construct formself a quadrant, by which he could take some simple astronomical observations. He furtlculated a table to give the sun's altitudes at different hours, and thus displayed those tasr practical astronomy which he lived to develop so greatly. It appears that these scientificudies were discountenanced by his father, who designed that his son should follow ausiness career. Flamsteed's natural inclination, however, forced him to prosecutetronomical work, notwithstanding the impediments that lay in his path. Unfortunately, his

onstitutional delicacy seems to have increased, and he had just completed his eighteenthear, "when," to use his own words, "the winter came on and thrust me again into thehimney, whence the heat and the dryness of the preceding summer had happily once befothdrawn me. But, it not being a fit season for physic, it was thought fit to let me alone th




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nter, and try the skill of another physician on me in the spring."

appears that at this time a quack named Valentine Greatrackes, was reputed to havefected most astonishing cures in Ireland merely by the stroke of his hands, without the

pplication of any medicine whatever. Flamsteed's father, despairing of any remedy for his om the legitimate branch of the profession, despatched him to Ireland on August 26th, 16e being then, as recorded with astronomical accuracy, "nineteen years, six days, and eleve

ours old." The young astronomer, accompanied by a friend, arrived on a Tuesday atverpool but the wind not being favourable, they remained there till the following Friday,hen a shift of the wind to the east took place. They embarked accordingly on a vessel calle SUPPLY at noon, and on Saturday night came in sight of Dublin. Ere they could land,

owever, they were nearly being wrecked on Lambay Island. This peril safely passed, thereas a long delay for quarantine before they were at last allowed on shore. On Thursday,eptember 6th, they set out from Dublin, where they had been sojourning at the "Ship" HoDame Street, towards Assaune, where Greatrackes received his patients.

amsteed gives an interesting account of his travels in Ireland. They dined at Naas on thest day, and on September 8th they reached Carlow, a town which is described as one of tirest they saw on their journey. By Sunday morning, September 10th, having lost their waveral times, they reached Castleton, called commonly Four Mile Waters. Flamsteed inquirthe host in the inn where they might find a church, but was told that the minister lived

welve miles away, and that they had no sermon except when he came to receive his tithesnce a year, and a woman added that "they had plenty enough of everything necessaryxcept the word of God." The travellers accordingly went on to Cappoquin, which lies up thver Blackwater, on the road to Lismore, eight miles from Youghal. Thence they immediate

arted on foot to Assaune. About a mile from Cappoquin, and entering into the house of Mreatrackes, they saw him touch several patients, "whereof some were nearly cured, otherere on the mending hand, and some on whom his strokes had no effect." Flamsteed wasuched by the famous quack on the afternoon of September 11th, but we are hardlyrprised to hear his remark that "he found not his disease to stir." Next morning thetronomer came again to see Mr. Greatrackes, who had "a kind of majestical yet affableesence, and a composed carriage." Even after the third touching had been submitted to,

enefit seems to have been derived. We must, however record, to the credit of Mr.reatrackes, that he refused to accept any payment from Flamsteed, because he was a

ranger.

nding it useless to protract his stay any longer, Flamsteed and his friend set out on theirturn to Dublin. In the course of his journey he seems to have been much impressed withonmel, which he describes as an "exceedingly pleasantly seated town." But in those days urney to Ireland was so serious an enterprise that when Flamsteed did arrive safely back erby after an absence of a month, he adds, "For God's providence in this journey, His name praised, Amen."




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s to the expected benefits to his health from the expedition we may quote his own words:n the winter following I was indifferent hearty, and my disease was not so violent as it usbe at that time formerly. But whether through God's mercy I received this through Mr.

reatrackes' touch, or my journey and vomiting at sea, I am uncertain; but, by somercumstances, I guess that I received a benefit from both."

is evident that by this time Flamsteed's interest in all astronomical matters had greatly

creased. He studied the construction of sun-dials, he formed a catalogue of seventy of thexed stars, with their places on the heavens, and he computed the circumstances of the soclipse which was to happen on June 22nd, 1666. It is interesting to note that even in thosays the doctrines of the astrologers still found a considerable degree of credence, andamsteed spent a good deal of his time in astrological studies and computations. Hevestigated the methods of casting a nativity, but a suspicion, or, indeed, rather more thanspicion, seems to have crossed his mind as to the value of these astrological predictions,

e says in fine, "I found astrology to give generally strong conjectural hints, not perfecteclarations."

l this time, however, the future Astronomer Royal was steadily advancing in astronomicalquiries of a recondite nature. He had investigated the obliquity of the ecliptic with extremre, so far as the circumstances of astronomical observation would at that time permit. He

ad also sought to discover the sun's distance from the earth in so far as it could be obtainy determining when the moon was exactly half illuminated, and he had measured, with mccuracy, the length of the tropical year. It will thus be seen that, even at the age of twentamsteed had made marked progress, considering how much his time had been interferedth by ill-health.

ther branches of astronomy began also to claim his attention. We learn that in 1669 and670 he compared the planets Jupiter and Mars with certain fixed stars near which theyassed. His instrumental means, though very imperfect, were still sufficient to enable him teasure the intervals on the celestial sphere between the planets and the stars. As the plathe stars were known, Flamsteed was thus able to obtain the places of the planets. This bstantially the way in which astronomers of the present day still proceed when they desirdetermine the places of the planets, inasmuch as, directly or indirectly those places are

ways obtained relatively to the fixed stars. By his observations at this early period, Flamst

as, it is true, not able to obtain any great degree of accuracy; he succeeded, however, inoving that the tables by which the places of the planets were ordinarily given were not tolied upon.

amsteed's labours in astronomy and in the allied branches of science were now becomingenerally known, and he gradually came to correspond with many distinguished men of arning. One of the first occasions which brought the talents of the young astronomer intome was the publication of some calculations concerning certain astronomical phenomenahich were to happen in the year 1670. In the monthly revolution of the moon its disc pass




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ver those stars which lie along its track. The disappearance of a star by the interposition oe moon is called an "occultation." Owing to the fact that our satellite is comparatively nea

s, the position which the moon appears to occupy on the heavens varies from different pathe earth, it consequently happens that a star which would be occulted to an observer in

ne locality, would often not be occulted to an observer who was situated elsewhere. Evenhen an occultation is visible from both places, the times at which the star disappears fromew will, generally speaking, be different. Much calculation is therefore necessary to decide

e circumstances under which the occultations of stars may be visible from any particularation. Having a taste for such computations, Flamsteed calculated the occultations whichere to happen in the year 1670, it being the case that several remarkable stars would beassed over by the moon during this year. Of course at the present time, we find suchformation duly set forth in the NAUTICAL ALMANAC, but a couple of centuries ago there wo such source of astronomical knowledge as is now to be found in that invaluable publicathich astronomers and navigators know so well. Flamsteed accordingly sent the results of ork to the President of the Royal Society. The paper which contained them was received vvourably, and at once brought Flamsteed into notice among the most eminent members o

at illustrious body, one of whom, Mr. Collins, became through life his faithful friend andonstant correspondent. Flamsteed's father was naturally gratified with the remarkable notihich his son was receiving from the great and learned; accordingly he desired him to go tondon, that he might make the personal acquaintance of those scientific friends whom head only known by correspondence previously. Flamsteed was indeed glad to avail himself is opportunity. Thus he became acquainted with Dr. Barrow, and especially with Newton,ho was then Lucasian Professor of Mathematics at Cambridge. It seems to have been inonsequence of this visit to London that Flamsteed entered himself as a member of Jesusollege, Cambridge. We have but little information as to his University career, but at all eve

e took his degree of M.A. on June 5th, 1674.

p to this time it would seem that Flamsteed had been engaged, to a certain extent, in theusiness carried on by his father. It is true that he does not give any explicit details, yet thee frequent references to journeys which he had to take on business matters. But the time

ow approached when Flamsteed was to start on an independent career, and it appears thae took his degree in Cambridge with the object of entering into holy orders, so that he migttle in a small living near Derby, which was in the gift of a friend of his father, and wouldthe disposal of the young astronomer. This scheme was, however, not carried out, but

amsteed does not tell us why it failed, his only remark being, that "the good providence ood that had designed me for another station ordered it otherwise."

r Jonas Moore, one of the influential friends whom Flamsteed's talents had attracted, seemhave procured for him the position of king's astronomer, with a salary of 100 pounds per

nnum. A larger salary appears to have been designed at first for this office, which was noweing newly created, but as Flamsteed was resolved on taking holy orders, a lesser salary whis case deemed sufficient. The building of the observatory, in which the first Astronome

oyal was to be installed, seems to have been brought about, or, at all events, its progress




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as accelerated, in a somewhat curious manner.

Frenchman, named Le Sieur de S. Pierre, came over to London to promulgate a scheme fscovering longitudes, then a question of much importance. He brought with himtroductions to distinguished people, and his mission attracted a great deal of attention. Thoposals which he made came under Flamsteed's notice, who pointed out that theenchman's projects were quite inapplicable in the present state of astronomical science,

asmuch as the places of the stars were not known with the degree of accuracy which woue necessary if such methods were to be rendered available. Flamsteed then goes on toy:--"I heard no more of the Frenchman after this; but was told that my letters had beenown King Charles. He was startled at the assertion of the fixed stars' places being false ine catalogue, and said, with some vehemence, he must have them anew observed,

xamined, and corrected, for the use of his seamen."

he first question to be settled was the site for the new observatory. Hyde Park and Chelseollege were both mentioned as suitable localities, but, at Sir Christopher Wren's suggestio

reenwich Hill was finally resolved upon. The king made a grant of five hundred pounds of oney. He gave bricks from Tilbury Fort, while materials, in the shape of wood, iron, andad, were available from a gatehouse demolished in the Tower. The king also promisedhatever further material aid might be shown to be necessary. The first stone of the Royalbservatory was laid on August 10th, 1675, and within a few years a building was erected hich the art of modern practical astronomy was to be created. Flamsteed strove withxtraordinary diligence, and in spite of many difficulties, to obtain a due provision of tronomical instruments, and to arrange for the carrying on of his observations.otwithstanding the king's promises, the astronomer was, however, but scantily provided w

eans, and he had no assistants to help him in his work. It follows that all the observations well as the reductions, and, indeed, all the incidental work of the observatory, had to berried on by himself alone. Flamsteed, as we have seen, had, however, many staunchends. Sir Jonas Moore in particular at all times rendered him most valuable assistance, an

ncouraged him by the warm sympathy and keen interest which he showed in astronomy. Tork of the first Astronomer Royal was frequently interrupted by recurrent attacks of theomplaints to which we have already referred. He says himself that "his distempers stick soose that that he cannot remove them," and he lost much time by prostration fromeadaches, as well as from more serious affections.

he year 1678 found him in the full tide of work in his observatory. He was specially engagn the problem of the earth's motion, which he sought to derive from observations of the snd of Venus. But this, as well as many other astronomical researches which he undertook,ere only subsidiary to that which he made the main task of his life, namely, the formationcatalogue of fixed stars. At the time when Flamsteed commenced his career, the onlyvailable catalogue of fixed stars was that of Tycho Brahe. This work had been published ae commencement of the seventeenth century, and it contained about a thousand stars. T

ositions assigned to these stars, though obtained with wonderful skill, considering the man




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fficulties under which Tycho laboured, were quite inaccurate when judged by our modernandards. Tycho's instruments were necessarily most rudely divided, and he had, of courseo telescopes to aid him. Consequently it was merely by a process of sighting that he couldbtain the places of the stars. It must further be remembered that Tycho had no clocks, ano micrometers. He had, indeed, but little correct knowledge of the motions of the heavenlyodies to guide him. To determine the longitudes of a few principal stars he conceived thegenious idea of measuring by day the position of Venus with respect to the sun, an

bservation which the exceptional brightness of this planet rendered possible withoutlescopic aid, and then by night he observed the position of Venus with regard to the stars

has been well remarked by Mr. Baily, in his introduction to the "British Catalogue of Starsat "Flamsteed's observations, by a fortunate combination of circumstances, commenced a

ew and a brilliant era. It happened that, at that period, the powerful mind of Newton wasrected to this subject; a friendly intercourse then existed between these two distinguished

haracters; and thus the first observations that could lay any claim to accuracy were at oncought in aid of those deep researches in which our illustrious geometer was then engaged

he first edition of the `Principia' bears testimony to the assistance afforded by Flamsteed tewton in these inquiries; although the former considers that the acknowledgment is not somple as it ought to have been."

though Flamsteed's observations can hardly be said to possess the accuracy of those madmore recent times, when instruments so much superior to his have been available, yet th

ossess an interest of a special kind from their very antiquity. This circumstance renders thparticular importance to the astronomer, inasmuch as they are calculated to throw light oe proper motions of the stars. Flamsteed's work may, indeed, be regarded as the origin o

bsequent catalogues, and the nomenclature which he adopted, though in some respects n hardly be said to be very defensible, is, nevertheless, that which has been adopted by absequent astronomers. There were also a great many errors, as might be expected in aork of such extent, composed almost entirely of numerical detail. Many of these errors haeen corrected by Baily himself, the assiduous editor of "Flamsteed's Life and Works," foramsteed was so harassed from various causes in the latter part of his life, and was sobject to infirmities all through his career, that he was unable to revise his computations we care that would have been necessary. Indeed, he observed many additional stars which

ever included in the British Catalogue. It is, as Baily well remarks, "rather a matter of

tonishment that he accomplished so much, considering his slender means, his weak framnd the vexations which he constantly experienced."

amsteed had the misfortune, in the latter part of his life, to become estranged from his mminent scientific contemporaries. He had supplied Newton with places of the moon, at thegent solicitation of the author of the "Principia," in order that the lunar theory should berefully compared with observation. But Flamsteed appears to have thought that in Newtorther request for similar information, he appeared to be demanding as a right that whichamsteed considered he was only called upon to render as a favour. A considerable dispute




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ew out of this matter, and there are many letters and documents, bearing on the difficulthich subsequently arose, that are not, perhaps, very creditable to either party.

otwithstanding his feeble constitution, Flamsteed lived to the age of seventy-three, his deccurring on the last day of the year 1719.

HALLEY.

aac Newton was just fourteen years of age when the birth of Edmund Halley, who wasestined in after years to become Newton's warmly attached friend, and one of his mostustrious scientific contemporaries, took place. There can be little doubt that the fame as atronomer which Halley ultimately acquired, great as it certainly was, would have been eveeater still had it not been somewhat impaired by the misfortune that he had to shine in thme sky as that which was illumined by the unparalleled genius of Newton.

dmund Halley was born at Haggerston, in the Parish of St. Leonard's, Shoreditch, on Octo9th, 1656. His father, who bore the same name as his famous son, was a soap-boiler ininchester Street, London, and he had conducted his business with such success that he

ccumulated an ample fortune. I have been unable to obtain more than a very few particulth respect to the early life of the future astronomer. It would, however, appear that from

oyhood he showed considerable aptitude for the acquisition of various kinds of learning, ae also had some capacity for mechanical invention. Halley seems to have received a soundducation at St. Paul's School, then under the care of Dr. Thomas Gale.

ere, the young philosopher rapidly distanced his competitors in the various branches of dinary school instruction. His superiority was, however, most conspicuous in mathematicaudies, and, as a natural development of such tastes, we learn that by the time he had lefthool he had already made good progress in astronomy. At the age of seventeen he was

ntered as a commoner at Queen's College, Oxford, and the reputation that he brought witm to the University may be inferred from the remark of the writer of "Athenae Oxonienseat Halley came to Oxford with skill in Latin, Greek, and Hebrew, and such a knowledge of

eometry as to make a complete dial." Though his studies were thus of a some-whatultifarious nature, yet it is plain that from the first his most favourite pursuit was astronoms earliest efforts in practical observation were connected with an eclipse which he observe

om his father's house in Winchester Street. It also appears that he had studied theoreticaanches of astronomy so far as to be conversant with the application of mathematics to

omewhat abstruse problems.

p to the time of Kepler, philosophers had assumed almost as an axiom that the heavenlyodies must revolve in circles and that the motion of the planet around the orbit which itescribed must be uniform. We have already seen how that great philosopher, after veryersevering labour, succeeded in proving that the orbits of the planets were not circles, but




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om Tycho Brahe's observatory at Uraniborg, in Denmark, could be said to have beenoperly observed. There was, no doubt, a rumour that a Dutchman had observed southernars from the island of Sumatra, and certain stars were indicated in the southern heavens celestial globe. On examination, however, Halley found that no reliance could be placed oe results which had been obtained, so that practically the field before him may be said to

ave been unworked.

the age of twenty, without having even waited to take that degree at the university whice authorities would have been glad to confer on so promising an undergraduate, this ardeudent of Nature sought his father's permission to go to the southern hemisphere for theurpose of studying the stars which lie around the southern pole. His father possessed theecessary means, and he had likewise the sagacity to encourage the young astronomer. Heas indeed most anxious to make every thing as easy as possible for so hopeful a son. Heovided him with an allowance of 300 pounds a year, which was regarded as a veryunificent provision in those days. Halley was also furnished with letters of recommendatioom King Charles II., as well as from the directors of the East India Company. He accordin

t sail with his instruments in the year 1676, in one of the East India Company's ships, fore island of St. Helena, which he had selected as the scene of his labours.

ter an uneventful voyage of three months, the astronomer landed on St. Helena, with hisxtant of five and a half feet radius, and a telescope 24 feet long, and forthwith plunged wdour into his investigation of the southern skies. He met, however, with one very

onsiderable disappointment. The climate of this island had been represented to him as movourable for astronomical observation; but instead of the pure blue skies he had been led

xpect, he found that they were almost always more or less clouded, and that rain was

equent, so that his observations were very much interrupted. On this account he onlymained at St. Helena for a single year, having, during that time, and in spite of manyfficulties, accomplished a piece of work which earned for him the title of "our southernycho." Thus did Halley establish his fame as an astronomer on the same lonely rock in midlantic, which nearly a century and a-half later became the scene of Napoleon's

mprisonment, when his star, in which he believed so firmly, had irretrievably set.

n his return to England, Halley prepared a map which showed the result of his labours, ane presented it to the king, in 1677. Like his great predecessor Tycho, Halley did not

together disdain the arts of the courtier, for he endeavoured to squeeze a new constellatito the group around the southern pole which he styled "The Royal Oak," adding aescription to the effect that the incidents of which "The Royal Oak" was a symbol were of fficient importance to be inscribed on the surface of the heavens.

here is reason to think that Charles II. duly appreciated the scientific renown which one ofs subjects had achieved, and it was probably through the influence of the king that Halleyas made a Master of Arts at Oxford on November 18th, 1678. Special reference was maden the occasion to his observations at St. Helena, as evidence of unusual attainments in




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athematics and astronomy. This degree was no small honour to such a young man, who, e have seen, quitted his university before he had the opportunity of graduating in thedinary manner.

n November 30th, in the same year, the astronomer received a further distinction in beingected a Fellow of the Royal Society. From this time forward he took a most active part in fairs of the Society, and the numerous papers which he read before it form a very valuab

art of that notable series of volumes known as the "Philosophical Transactions." He wasbsequently elected to the important office of secretary to the Royal Society, and hescharged the duties of his post until his appointment to Greenwich necessitated hissignation.

ithin a year of Halley's election as a Fellow of the Royal Society, he was chosen by theociety to represent them in a discussion which had arisen with Hevelius. The nature of thisscussion, or rather the fact that any discussion should have been necessary, may seemrange to modern astronomers, for the point is one on which it would now seem impossibl

r there to be any difference of opinion. We must, however, remember that the days of alley were, comparatively speaking, the days of infancy as regards the art of astronomicalbservation, and issues that now seem obvious were often, in those early times, the occasigrave and anxious consideration. The particular question on which Halley had to represee Royal Society may be simply stated. When Tycho Brahe made his memorablevestigations into the places of the stars, he had no telescopes to help him. The famousstruments at Uraniborg were merely provided with sights, by which the telescope wasointed to a star on the same principle as a rifle is sighted for a target. Shortly after Tycho'me, Galileo invented the telescope. Of course every one admitted at once the extraordinar

dvantages which the telescope had to offer, so far as the mere question of the visibility ofbjects was concerned. But the bearing of Galileo's invention upon what we may describe ae measuring part of astronomy was not so immediately obvious. If a star be visible to the

naided eye, we can determine its place by such instruments as those which Tycho used, inhich no telescope is employed. We can, however, also avail ourselves of an instrument inhich we view the star not directly but through the intervention of the telescope. Can theace of the star be determined more accurately by the latter method than it can when thelescope is dispensed with? With our present knowledge, of course, there is no doubt aboue answer; every one conversant with instruments knows that we can determine the place

star far more accurately with the telescope than is possible by any mere sighting apparatfact an observer would be as likely to make an error of a minute with the sighting

pparatus in Tycho's instrument, as he would be to make an error of a second with theodern telescope, or, to express the matter somewhat differently, we may say, speaking qenerally, that the telescopic method of determining the places of the stars does not lead torors more than one-sixtieth part as great as which are unavoidable when we make use ofycho's method.

ut though this is so apparent to the modern astronomer, it was not at all apparent in the




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nd to take so large and comprehensive a view of the subject as he appears to have done."692, Halley explained his theory of terrestrial magnetism, and begged captains of ships toke observations of the variations of the compass in all parts of the world, and to

ommunicate them to the Royal Society, "in order that all the facts may be readily availableose who are hereafter to complete this difficult and complicated subject."

he extent to which Halley was in advance of his contemporaries, in the study of terrestrial

agnetism, may be judged from the fact that the subject was scarcely touched after his timl the year 1811. The interest which he felt in it was not of a merely theoretical kind, nor wone which could be cultivated in an easy-chair. Like all true investigators, he longed tobmit his theory to the test of experiment, and for that purpose Halley determined to obsee magnetic variation for himself. He procured from King William III. the command of a

essel called the "Paramour Pink," with which he started for the South Seas in 1694. Thisarticular enterprise was not, however, successful; for, on crossing the line, some of his mell sick and one of his lieutenants mutinied, so that he was obliged to return the following

ear with his mission unaccomplished. The government cashiered the lieutenant, and Halley

aving procured a second smaller vessel to accompany the "Paramour Pink," started onceore in September, 1699. He traversed the Atlantic to the 52nd degree of southern latitudeeyond which his further advance was stopped. "In these latitudes," he writes to say, "we fwith great islands of ice of so incredible height and magnitude, that I scarce dare write moughts of it."

n his return in 1700, Halley published a general chart, showing the variation of the compathe different places which he had visited. On these charts he set down lines connectingose localities at which the magnetic variation was identical. He thus set an example of the

aphic representation of large masses of complex facts, in such a manner as to appeal atnce to the eye, a method of which we make many applications in the present day.

ut probably the greatest service which Halley ever rendered to human knowledge was theare in which he took in bringing Newton's "Principia" before the world. In fact, as Dr.aisher, writing in 1888, has truly remarked, "but for Halley the 'Principia' would not have

xisted."

was a visit from Halley in the year 1684 which seems to have first suggested to Newton t

ea of publishing the results of his investigations on gravitation. Halley, and other scientificontemporaries, had no doubt some faint glimmering of the great truth which only Newtonenius was able fully to reveal. Halley had indeed shown how, on the assumptions that theanets move in circular orbits round the sun, and that the squares of their periodic times aoportional to the cubes of their mean distances, it may be proved that the force acting on

ach planet must vary inversely as the square of its distance from the sun. Since, however,ach of the planets actually moves in an ellipse, and therefore, at continually varying distanom the sun, it becomes a much more difficult matter to account mathematically for theody's motions on the supposition that the attractive force varies inversely as the square of




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e distance. This was the question with which Halley found himself confronted, but which athematical abilities were not adequate to solve. It would seem that both Hooke and Sirhristopher Wren were interested in the same problem; in fact, the former claimed to haverived at a solution, but declined to make known his results, giving as an excuse his desireat others having tried and failed might learn to value his achievements all the more. Halle

owever, confessed that his attempts at the solution were unsuccessful, and Wren, in ordencourage the other two philosophers to pursue the inquiry, offered to present a book of fo

illings value to either of them who should in the space of two months bring him a convincoof of it. Such was the value which Sir Christopher set on the Law of Gravitation, uponhich the whole fabric of modern astronomy may be said to stand.

nding himself unequal to the task, Halley went down to Cambridge to see Newton on thebject, and was delighted to learn that the great mathematician had already completed thvestigation. He showed Halley that the motions of all the planets could be completelyccounted for on the hypothesis of a force of attraction directed towards the sun, which vaversely as the square of the distance from that body.

alley had the genius to perceive the tremendous importance of Newton's researches, and eased not to urge upon the recluse man of science the necessity for giving his newscoveries publication. He paid another visit to Cambridge with the object of learning moreth regard to the mathematical methods which had already conducted Newton to suchblime truths, and he again encouraged the latter both to pursue his investigations, and tove some account of them to the world. In December of the same year Halley had theatification of announcing to the Royal Society that Newton had promised to send that bod

aper containing his researches on Gravitation.

seems that at this epoch the finances of the Royal Society were at a very low ebb. Thismpecuniosity was due to the fact that a book by Willoughby, entitled "De Historia Piscium,"ad been recently printed by the society at great expense. In fact, the coffers were so lowat they had some difficulty in paying the salaries of their permanent officials. It appears te public did not care about the history of fishes, or at all events the volume did not meetth the ready demand which was expected for it. Indeed, it has been recorded that whenalley had undertaken to measure the length of a degree of the earth's surface, at the requthe Royal Society, it was ordered that his expenses be defrayed either in 50 pounds

erling, or in fifty books of fishes. Thus it happened that On June 2nd, the Council, after duonsideration of ways and means in connection with the issue of the Principia," ordered thaalley should undertake the business of looking after the book and printing it at his ownharge," which he engaged to do.

was, as we have elsewhere mentioned, characteristic of Newton that he detestedontroversies, and he was, in fact, inclined to suppress the third book of the "Principia"together rather than have any conflict with Hooke with respect to the discoveries there




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nunciated. He also thought of changing the name of the work to De Motu Corporum Libriuo, but upon second thoughts, he retained the original title, remarking, as he wrote toalley, "It will help the sale of the book, which I ought not to diminish, now it is yours," antence which shows conclusively, if further proof were necessary, that Halley had assumee responsibility of its publication.

alley spared no pains in pushing forward the publication of his illustrious friend's great wo

o that in the same year he was in a position to present a complete copy to King James II.,th a proper discourse of his own. Halley also wrote a set of Latin hexameters in praise of ewton's genius, which he printed at the beginning of the work. The last line of this specimHalley's poetic muse may be thus rendered: Nor mortals nearer may approach the gods.

he intimate friendship between the two greatest astronomers of the time continued withoterruption till the death of Newton. It has, indeed, been alleged that some serious cause otrangement arose between them. There is, however, no satisfactory ground for thisatement; indeed, it may be regarded as effectually disposed of by the fact that, in the yea

727, Halley took up the defence of his friend, and wrote two learned papers in support of ewton's "System of Chronology," which had been seriously attacked by a certain ecclesiasis quite evident to any one who has studied these papers that Halley's friendship for Newas as ardent as ever.

he generous zeal with which Halley adopted and defended the doctrines of Newton withgard to the movements of the celestial bodies was presently rewarded by a brilliantscovery, which has more than any of his other researches rendered his name a familiar onastronomers. Newton, having explained the movement of the planets, was naturally led t

rn his attention to comets. He perceived that their journeyings could be completelyccounted for as consequences of the attraction of the sun, and he laid down the principleshich the orbit of a comet could be determined, provided that observations of its positionsere obtained at three different dates. The importance of these principles was by no oneore quickly recognised than by Halley, who saw at once that it provided the means of etecting something like order in the movements of these strange wanderers. The doctrineravitation seemed to show that just as the planets revolved around the sun in ellipses, soso must the comets. The orbit, however, in the case of the comet, is so extremely elongaat the very small part of the elliptic path within which the comet is both near enough and

ight enough to be seen from the earth, is indistinguishable from a parabola. Applying theinciples, Halley thought it would be instructive to study the movements of certain bright

omets, concerning which reliable observations could be obtained. At the expense of muchbour, he laid down the paths pursued by twenty-four of these bodies, which had appeareetween the years 1337 and 1698. Amongst them he noticed three, which followed tracks sosely resembling each other, that he was led to conclude the so called three comets couldnly have been three different appearances of the same body. The first of these occurred in531, the second was seen by Kepler in 1607, and the third by Halley himself in 1682. Thesates suggested that the observed phenomena might be due to the successive returns of o




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nd the same comet after intervals of seventy-five or seventy-six years. On the furtherxamination of ancient records, Halley found that a comet had been seen in the year 1456,ate, it will be observed, seventy-five years before 1531. Another had been observed sevenx years earlier than 1456, viz., in 1380, and another seventy-five years before that, in 130

s Halley thus found that a comet had been recorded on several occasions at intervals of venty-five or seventy-six years, he was led to the conclusion that these several apparition

lated to one and the same object, which was an obedient vassal of the sun, performing accentric journey round that luminary in a period of seventy-five or seventy-six years. Toalise the importance of this discovery, it should be remembered that before Halley's time

omet, if not regarded merely as a sign of divine displeasure, or as an omen of intendingsaster, had at least been regarded as a chance visitor to the solar system, arriving no onenew whence, and going no one knew whither.

supreme test remained to be applied to Halley's theory. The question arose as to the datehich this comet would be seen again. We must observe that the question was complicated

e fact that the body, in the course of its voyage around the sun, was exposed to thecessant disturbing action produced by the attraction of the several planets. The cometerefore, does not describe a simple ellipse as it would do if the attraction of the sun weree only force by which its movement were controlled. Each of the planets solicits the comedepart from its track, and though the amount of these attractions may be insignificant in

omparison with the supreme controlling force of the sun, yet the departure from the ellipsuite sufficient to produce appreciable irregularities in the comet's movement. At the timehen Halley lived, no means existed of calculating with precision the effect of the disturbancomet might experience from the action of the different planets. Halley exhibited his usua

tronomical sagacity in deciding that Jupiter would retard the return of the comet to somextent. Had it not been for this disturbance the comet would apparently have been due in757 or early in 1758. But the attraction of the great planet would cause delay, so that Hallsigned, for the date of its re-appearance, either the end of 1758 or the beginning of 1759alley knew that he could not himself live to witness the fulfilment of his prediction, but heys: "If it should return, according to our predictions, about the year 1758, impartial postell not refuse to acknowledge that this was first discovered by an Englishman." This was,deed, a remarkable prediction of an event to occur fifty-three years after it had beentered. The way in which it was fulfilled forms one of the most striking episodes in the hist

astronomy. The comet was first seen on Christmas Day, 1758, and passed through itsearest point to the sun on March 13th, 1759. Halley had then been lying in his grave forventeen years, yet the verification of his prophecy reflects a glory on his name which willuse it to live for ever in the annals of astronomy. The comet paid a subsequent visit in 18

nd its next appearance is due about 1910.

alley next entered upon a labour which, if less striking to the imagination than his discoveth regard to comets, is still of inestimable value in astronomy. He undertook a series of vestigations with the object of improving our knowledge of the movements of the planets




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his task was practically finished in 1719, though the results of it were not published untilter his death in 1749. In the course of it he was led to investigate closely the motion of enus, and thus he came to recognise for the first time the peculiar importance whichtaches to the phenomenon of the transit of this planet across the sun. Halley saw that theansit, which was to take place in the year 1761, would afford a favourable opportunity foretermining the distance of the sun, and thus learning the scale of the solar system. Heedicted the circumstances of the phenomenon with an astonishing degree of accuracy,

onsidering his means of information, and it is unquestionably to the exertions of Halley inging the importance of the matter upon astronomers that we owe the unexampled degreterest taken in the event, and the energy which scientific men exhibited in observing it. Tustrious astronomer had no hope of being himself a witness of the event, for it could notappen till many years after his death. This did not, however, diminish his anxiety to imprepon those who would then be alive, the importance of the occurrence, nor did it lead him eglect anything which might contribute to the success of the observations. As we now knoalley rather over-estimated the value of the transit of Venus, as a means of determining tolar distance. The fact is that the circumstances are such that the observation of the time

ontact between the edge of the planet and the edge of the sun cannot be made with theccuracy which he had expected.

1691, Halley became a candidate for the Savilian Professorship of Astronomy at Oxford. as not, however, successful, for his candidature was opposed by Flamsteed, the Astronomoyal of the time, and another was appointed. He received some consolation for this particsappointment by the fact that, in 1696, owing to Newton's friendly influence, he wasppointed deputy Controller of the Mint at Chester, an office which he did not retain for lon it was abolished two years later. At last, in 1703, he received what he had before vainly

ought, and he was appointed to the Savilian chair.

s observations of the eclipse of the sun, which occurred in 1715, added greatly to Halley'sputation. This phenomenon excited special attention, inasmuch as it was the first total

clipse of the sun which had been visible in London since the year 1140. Halley undertook tecessary calculations, and predicted the various circumstances with a far higher degree ofecision than the official announcement. He himself observed the phenomenon from theoyal Society's rooms, and he minutely describes the outer atmosphere of the sun, nownown as the corona; without, however, offering an opinion as to whether it was a solar or

nar appendage.

last Halley was called to the dignified office which he of all men was most competent to n February 9th, 1720, he was appointed Astronomer Royal in succession to Flamsteed. Heund things at the Royal Observatory in a most unsatisfactory state. Indeed, there were nostruments, nor anything else that was movable; for such things, being the property of amsteed, had been removed by his widow, and though Halley attempted to purchase fromat lady some of the instruments which his predecessor had employed, the unhappy persofferences which had existed between him and Flamsteed, and which, as we have already




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en, prevented his election as Savilian Professor of Astronomy, proved a bar to theegotiation. Greenwich Observatory wore a very different appearance in those days, from thich the modern visitor, who is fortunate enough to gain admission, may now behold. Notnly did Halley find it bereft of instruments, we learn besides that he had no assistants, andas obliged to transact the whole business of the establishment single-handed.

1721, however, he obtained a grant of 500 pounds from the Board of Ordnance, and

ccordingly a transit instrument was erected in the same year. Some time afterwards heocured an eight-foot quadrant, and with these instruments, at the age of sixty-four, he

ommenced a series of observations on the moon. He intended, if his life was spared, toontinue his observations for a period of eighteen years, this being, as astronomers know, aery important cycle in connection with lunar movements. The special object of this vastndertaking was to improve the theory of the moon's motion, so that it might serve moreccurately to determine longitudes at sea. This self-imposed task Halley lived to carry to accessful termination, and the tables deduced from his observations, and published after h

eath, were adopted almost universally by astronomers, those of the French nation being t

nly exception.

hroughout his life Halley had been singularly free from illness of every kind, but in 1737 head a stroke of paralysis. Notwithstanding this, however, he worked diligently at his telescol 1739, after which his health began rapidly to give way. He died on January 14th, 1742, e eighty-sixth year of his age, retaining his mental faculties to the end. He was buried in

emetery of the church of Lee in Kent, in the same grave as his wife, who had died five yeaeviously. We are informed by Admiral Smyth that Pond, a later Astronomer Royal, wasterwards laid in the same tomb.

alley's disposition seems to have been generous and candid, and wholly free from anythinke jealousy or rancour. In person he was rather above the middle height, and slight in buis complexion was fair, and he is said to have always spoken, as well as acted, withncommon sprightliness. In the eloge pronounced upon him at the Paris Academie Desciences, of which Halley had been made a member in 1719 it was said, "he possessed all tualifications which were necessary to please princes who were desirous of instruction, witheat extent of knowledge and a constant presence of mind; his answers were ready, and ae same time pertinent, judicious, polite and sincere."

hus we find that Peter the Great was one of his most ardent admirers. He consulted thetronomer on matters connected with shipbuilding, and invited him to his own table. Butalley possessed nobler qualifications than the capacity of pleasing Princes. He was able toxcite and to retain the love and admiration of his equals. This was due to the warmth of htachments, the unselfishness of his devotion to his friends, and to a vein of gaiety and goumour which pervaded all his conversation.




Great Astronomers

BRADLEY.

mes Bradley was descended from an ancient family in the county of Durham. He was bor1692 or 1693, at Sherbourne, in Gloucestershire, and was educated in the Grammar SchoNorthleach. From thence he proceeded in due course to Oxford, where he was admitted

ommoner at Balliol College, on March 15th, 1711. Much of his time, while an undergraduatas passed in Essex with his maternal uncle, the Rev. James Pound, who was a well-known

an of science and a diligent observer of the stars. It was doubtless by intercourse with hisncle that young Bradley became so expert in the use of astronomical instruments, but the

mmortal discoveries he subsequently made show him to have been a born astronomer.

he first exhibition of Bradley's practical skill seems to be contained in two observations whe made in 1717 and 1718. They have been published by Halley, whose acuteness had ledm to perceive the extraordinary scientific talents of the young astronomer. Anotherustration of the sagacity which Bradley manifested, even at the very commencement of htronomical career, is contained in a remark of Halley's, who says: Dr. Pound and his

ephew, Mr. Bradley, did, myself being present, in the last opposition of the sun and Mars tay demonstrate the extreme minuteness of the sun's parallax, and that it was not more th

welve seconds nor less than nine seconds." To make the significance of this plain, it shouldbserved that the determination of the sun's parallax is equivalent to the determination of tstance from the earth to the sun. At the time of which we are now writing, this very

mportant unit of celestial measurement was only very imperfectly known, and thebservations of Pound and Bradley may be interpreted to mean that, from their observationey had come to the conclusion that the distance from the earth to the sun must be morean 94 millions of miles, and less than 125 millions. We now, of course, know that they we

ot exactly right, for the true distance of the sun is about 93 millions of miles. We cannot,owever, but think that it was a very remarkable approach for the veteran astronomer and illiant nephew to make towards the determination of a magnitude which did not become

ccurately known till fifty years later.

mong the earliest parts of astronomical work to which Bradley's attention was directed, wee eclipses of Jupiter's satellites. These phenomena are specially attractive inasmuch as thn be so readily observed, and Bradley found it extremely interesting to calculate the timehich the eclipses should take place, and then to compare his observations with the predict

mes. From the success that he met with in this work, and from his other labours, Bradley'sputation as an astronomer increased so greatly that on November the 6th, 1718, he wasected a Fellow of the Royal Society.

p to this time the astronomical investigations of Bradley had been more those of an amatean of a professional astronomer, and as it did not at first seem likely that scientific work ould lead to any permanent provision, it became necessary for the youthful astronomer tohoose a profession. It had been all along intended that he should enter the Church, thougr some reason which is not told us, he did not take orders as soon as his age would have




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xamination of his book of Calculations which is still extant.

he time was now approaching when Bradley was to make the first of those two greatscoveries by which his name has acquired a lustre that has placed him in the very foremonk of astronomical discoverers. As has been often the case in the history of science, the fthese great successes was attained while he was pursuing a research intended for a who

fferent purpose. It had long been recognised that as the earth describes a vast orbit, nea

wo hundred million miles in diameter, in its annual journey round the sun, the apparentaces of the stars should alter, to some extent, in correspondence with the changes in thearth's position. The nearer the star the greater the shift in its apparent place on the heavehich must arise from the fact that it was seen from different positions in the earth's orbit. ad been pointed out that these apparent changes in the places of the stars, due to theovement of the earth, would provide the means of measuring the distances of the stars. Aowever, these distances are enormously great in comparison with the orbit which the eartescribes around the sun, the attempt to determine the distances of the stars by the shift ineir positions had hitherto proved ineffectual. Bradley determined to enter on this research

nce again; he thought that by using instruments of greater power, and by makingeasurements of increased delicacy, he would be able to perceive and to measuresplacements which had proved so small as to elude the skill of the other astronomers whoad previously made efforts in the same direction. In order to simplify the investigation asuch as possible, Bradley devoted his attention to one particular star, Beta Draconis, whichappened to pass near his zenith. The object of choosing a star in this position was to avoide difficulties which would be introduced by refraction had the star occupied any other plathe heavens than that directly overhead.

e are still able to identify the very spot on which the telescope stood which was used in temorable research. It was erected at the house then occupied by Molyneux, on the weste

xtremity of Kew Green. The focal length was 24 feet 3 inches, and the eye- glass was 3 analf feet above the ground floor. The instrument was first set up on November 26th, 1725. ere had be any appreciable disturbance in the place of Beta Draconis in consequence of tovement of the earth around the sun, the star must appear to have the smallest latitudehen in conjunction with the sun, and the greatest when in opposition. The star passed theeridian at noon in December, and its position was particularly noticed by Molyneux on theird of that month. Any perceptible displacement by parallax--for so the apparent change i

osition, due to the earth's motion, is called--would would have made the star shift towardse north. Bradley, however, when observing it on the 17th, was surprised to find that the

pparent place of the star, so far from shifting towards the north, as they had perhaps hopwould, was found to lie a little more to the south than when it was observed before. He t

xtreme care to be sure that there was no mistake in his observation, and, true astronomere was, he scrutinized with the utmost minuteness all the circumstances of the adjustment s instruments. Still the star went to the south, and it continued so advancing in the samerection until the following March, by which time it had moved no less than twenty second

outh from the place which it occupied when the first observation was made. After a brief




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ause, in which no apparent movement was perceptible, the star by the middle of Aprilppeared to be returning to the north. Early in June it reached the same distance from thenith which it had in December. By September the star was as much as thirty-nine secondore to the north than it had been in March, then it returned towards the south, regaining ecember the same situation which it had occupied twelve months before.

his movement of the star being directly opposite to the movements which would have bee

e consequence of parallax, seemed to show that even if the star had any parallax its effecpon the apparent place were entirely masked by a much larger motion of a totally differenescription. Various attempts were made to account for the phenomenon, but they were noccessful. Bradley accordingly determined to investigate the whole subject in a moreorough manner. One of his objects was to try whether the same movements which he ha

bserved in one star were in any similar degree possessed by other stars. For this purpose t up a new instrument at Wanstead, and there he commenced a most diligent scrutiny ofe apparent places of several stars which passed at different distances from the zenith. Heund in the course of this research that other stars exhibited movements of a similar

escription to those which had already proved so perplexing. For a long time the cause of ese apparent movements seemed a mystery. At last, however, the explanation of thesemarkable phenomena dawned upon him, and his great discovery was made.

ne day when Bradley was out sailing he happened to remark that every time the boat wasd on a different tack the vane at the top of the boat's mast shifted a little, as if there had

een a slight change in the direction of the wind. After he had noticed this three or four time made a remark to the sailors to the effect that it was very strange the wind should alwaappen to change just at the moment when the boat was going about. The sailors, howeve

id there had been no change in the wind, but that the alteration in the vane was due to tct that the boat's course had been altered. In fact, the position of the vane was determinoth by the course of the boat and the direction of the wind, and if either of these weretered there would be a corresponding change in the direction of the vane. This meant, of ourse, that the observer in the boat which was moving along would feel the wind comingom a point different from that in which the wind appeared to be blowing when the boat wrest, or when it was sailing in some different direction. Bradley's sagacity saw in this

bservation the clue to the Difficulty which had so long troubled him.

had been discovered before the time of Bradley that the passage of light through space iot an instantaneous phenomenon. Light requires time for its journey. Galileo surmised thae sun may have reached the horizon before we see it there, and it was indeed sufficiently

bvious that a physical action, like the transmission of light, could hardly take place withoutquiring some lapse of time. The speed with which light actually travelled was, however, spid that its determination eluded all the means of experimenting which were available inose days. The penetration of Roemer had previously detected irregularities in the observe

mes of the eclipses of Jupiter's satellites, which were undoubtedly due to the interval whicht required for stretching across the interplanetary spaces. Bradley argued that as light c




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self, but is rather to be attributed to changes in the points from which the star's positions easured.

e may explain the matter in this way. As the earth is not a sphere, but has protuberant pathe equator, the attraction of the moon exercises on those protuberant parts a pulling ef

hich continually changes the direction of the earth's axis, and consequently the position oe pole must be in a state of incessant fluctuation. The pole to which the earth's axis point

n the sky is, therefore, slowly changing. At present it happens to lie near the Pole Star, bull not always remain there. It describes a circle around the pole of the Ecliptic, requiring

bout 25,000 years for a complete circuit. In the course of its progress the pole will graduaass now near one star and now near another, so that many stars will in the lapse of agesscharge the various functions which the present Pole Star does for us. In about 12,000ears, for instance, the pole will have come near the bright star, Vega. This movement of thole had been known for ages. But what Bradley discovered was that the pole, instead of escribing an uniform movement as had been previously supposed, followed a sinuous couow on one side and now on the other of its mean place. This he traced to the fluctuations

e moon's orbit, which undergoes a continuous change in a period of nineteen years. Thuse efficiency with which the moon acts on the protuberant mass of the earth varies, and the pole is caused to oscillate.

his subtle discovery, if perhaps in some ways less impressive than Bradley's earlierchievements of the detection of the aberration of light, is regarded by astronomers asstifying even in a higher degree to his astonishing care and skill as an observer, and justly

ntitles him to a unique place among the astronomers whose discoveries have been effectey consummate practical skill in the use of astronomical instruments.

f Bradley's private or domestic life there is but little to tell. In 1744, soon after he becamestronomer Royal, he married a daughter of Samuel Peach, of Chalford, in Gloucestershire.here was but one child, a daughter, who became the wife of her cousin, Rev. Samuel Peacctor of Compton, Beauchamp, in Berkshire.

adley's last two years of life were clouded by a melancholy depression of spirits, due to apprehension that he should survive his rational faculties. It seems, however, that the ill heeaded never came upon him, for he retained his mental powers to the close. He died on

3th July, 1762, aged seventy, and was buried at Michinghamton.

WILLIAM HERSCHEL.

illiam Herschel, one of the greatest astronomers that has ever lived, was born at Hanovern the 15th November, 1738. His father, Isaac Herschel, was a man evidently of considerabbility, whose life was devoted to the study and practice of music, by which he earned aomewhat precarious maintenance. He had but few worldly goods to leave to his children, b




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e more than compensated for this by bequeathing to them a splendid inheritance of geniuouches of genius were, indeed, liberally scattered among the members of Isaac's largemily, and in the case of his forth child, William, and of a sister several years younger, it wnited with that determined perseverance and rigid adherence to principle which enabledenius to fulfil its perfect work.

faithful chronicler has given us an interesting account of the way in which Isaac Herschel

ducated his sons; the narrative is taken from the recollections of one who, at the time we peaking of, was an unnoticed little girl five or six years old. She writes:--

My brothers were often introduced as solo performers and assistants in the orchestra at thourt, and I remember that I was frequently prevented from going to sleep by the livelyiticisms on music on coming from a concert. Often I would keep myself awake that I mighten to their animating remarks, for it made me so happy to see them so happy. But

enerally their conversation would branch out on philosophical subjects, when my brotherilliam and my father often argued with such warmth that my mother's interference becam

ecessary, when the names--Euler, Leibnitz, and Newton--sounded rather too loud for thepose of her little ones, who had to be at school by seven in the morning." The child whosminiscences are here given became afterwards the famous Caroline Herschel. The narratiher life, by Mrs. John Herschel, is a most interesting book, not only for the account it

ontains of the remarkable woman herself, but also because it provides the best picture weave of the great astronomer to whom Caroline devoted her life.

his modest family circle was, in a measure, dispersed at the outbreak of the Seven Years'ar in 1756. The French proceeded to invade Hanover, which, it will be remembered,

elonged at this time to the British dominions. Young William Herschel had already obtainee position of a regular performer in the regimental band of the Hanoverian Guards, and itas his fortune to obtain some experience of actual warfare in the disastrous battle of astenbeck. He was not wounded, but he had to spend the night after the battle in a ditchnd his meditations on the occasion convinced him that soldiering was not the professionxactly adapted to his tastes. We need not attempt to conceal the fact that he left hisgiment by the very simple but somewhat risky process of desertion. He had, it would seeadopt disguises to effect his escape. At all events, by some means he succeeded in eludi

etection and reached England in safety. It is interesting to have learned on good authority

at many years after this offence was committed it was solemnly forgiven. When Herschelad become the famous astronomer, and as such visited King George at Windsor, the King eir first meeting handed to him his pardon for deserting from the army, written out in duerm by his Majesty himself.

seems that the young musician must have had some difficulty in providing for hisaintenance during the first few years of his abode in England. It was not until he hadached the age of twenty-two that he succeeded in obtaining any regular appointment. Heas then made Instructor of Music to the Durham Militia. Shortly afterwards, his talents bei




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onstruction. Night after night, as soon as his musical labours were ended, his telescopes wought out, sometimes into the small back garden of his house at Bath, and sometimes inte street in front of his hall-door. It was characteristic of him that he was always

ndeavouring to improve his apparatus. He was incessantly making fresh mirrors, or tryingew lenses, or combinations of lenses to act as eye-pieces, or projecting alterations in theounting by which the telescope was supported. Such was his enthusiasm that his house, we told, was incessantly littered with the usual indications of the workman's presence, grea

the distress of his sister, who, at this time, had come to take up her abode with him andok after his housekeeping. Indeed, she complained that in his astronomical ardour he

ometimes omitted to take off, before going into his workshop, the beautiful lace ruffles whe wore while conducting a concert, and that consequently they became soiled with the pitmployed in the polishing of his mirrors.

his sister, who occupies such a distinct place in scientific history is the same little girl tohom we have already referred. From her earliest days she seems to have cherished aassionate admiration for her brilliant brother William. It was the proudest delight of her

hildhood as well as of her mature years to render him whatever service she could; no manience was ever provided with a more capable or energetic helper than William Herschelund in this remarkable woman. Whatever work had to be done she was willing to bear heare in it, or even to toil at it unassisted if she could be allowed to do so. She not onlyanaged all his domestic affairs, but in the grinding of the lenses and in the polishing of thirrors she rendered every assistance that was possible. At one stage of the very delicateperation of fashioning a reflector, it is necessary for the workman to remain with his hand e mirror for many hours in succession. When such labours were in progress, Caroline usesit by her brother, and enliven the time by reading stories aloud, sometimes pausing to f

m with a spoon while his hands were engaged on the task from which he could not desistmoment.

hen mathematical work had to be done Caroline was ready for it; she had taught herself fficient to enable her to perform the kind of calculations, not, perhaps, very difficult onesat Herschel's work required; indeed, it is not too much to say that the mighty life-work hich this man was enabled to perform could never have been accomplished had it not beer the self- sacrifice of this ever-loving and faithful sister. When Herschel was at the telescnight, Caroline sat by him at her desk, pen in hand, ready to write down the notes of the

bservations as they fell from her brother's lips. This was no insignificant toil. The telescopeas, of course, in the open air, and as Herschel not unfrequently continued his observationroughout the whole of a long winter's night, there were but few women who could have

ccomplished the task which Caroline so cheerfully executed. From dusk till dawn, when theky was clear, were Herschel's observing hours, and what this sometimes implied we canalise from the fact that Caroline assures us she had sometimes to desist because the ink

ctually frozen in her pen. The night's work over, a brief rest was taken, and while William s labours for the day to attend to, Caroline carefully transcribed the observations madeuring the night before, reduced all the figures and prepared everything in readiness for th




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bservations that were to follow on the ensuing evening.

ut we have here been anticipating a little of the future which lay before the greattronomer; we must now revert to the history of his early work, at Bath, in 1774, whenerschel's scrutiny of the skies first commenced with an instrument of his own manufactureor some few years he did not attain any result of importance; no doubt he made a fewteresting observations, but the value of the work during those years is to be found, not in

ny actual discoveries which were accomplished, but in the practice which Herschel obtainethe use of his instruments. It was not until 1782 that the great achievement took place b

hich he at once sprang into fame.

is sometimes said that discoveries are made by accident, and, no doubt, to a certain exteut only, I fancy to a very small extent, this statement may be true. It is, at all events, certat such lucky accidents do not often fall to the lot of people unless those people have donuch to deserve them. This was certainly the case with Herschel. He appears to have formproject for making a close examination of all the stars above a certain magnitude. Perhap

e intended to confine this research to a limited region of the sky, but, at all events, he seehave undertaken the work energetically and systematically. Star after star was brought te centre of the field of view of his telescope, and after being carefully examined was thensplaced, while another star was brought forward to be submitted to the same process. In eat majority of cases such observations yield really nothing of importance; no doubt evene smallest star in the heavens would, if we could find out all about it, reveal far more tha the astronomers that were ever on the earth have even conjectured. What we actuallyarn about the great majority of stars is only information of the most meagre description. We that the star is a little point of light, and we see nothing more.

the great review which Herschel undertook he doubtless examined hundreds, or perhapsousands of stars, allowing them to pass away without note or comment. But on an ever-emorable night in March, 1782, it happened that he was pursuing his task among the stathe Constellation of Gemini. Doubtless, on that night, as on so many other nights, one stter another was looked at only to be dismissed, as not requiring further attention. On the

vening in question, however, one star was noticed which, to Herschel's acute vision seemefferent from the stars which in so many thousands are strewn over the sky. A star proper

o called appears merely as a little point of light, which no increase of magnifying power wi

ver exhibit with a true disc. But there was something in the star-like object which Herschew that immediately arrested his attention and made him apply to it a higher magnifying

ower. This at once disclosed the fact that the object possessed a disc, that is, a definite,easurable size, and that it was thus totally different from any one of the hundreds andousands of stars which exist elsewhere in space. Indeed, we may say at once that this litt

bject was not a star at all; it was a planet. That such was its true nature was confirmed, alittle further observation, by perceiving that the body was shifting its place on the heavenlatively to the stars. The organist at the Octagon Chapel at Bath had, therefore, discovere

ew planet with his home-made telescope.




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can imagine some one will say, "Oh, there was nothing so wonderful in that; are not planways being discovered? Has not M. Palisa, for instance discovered about eighty of suchbjects, and are there not hundreds of them known nowadays?" This is, to a certain extentuite true. I have not the least desire to detract from the credit of those industrious and shghted astronomers who have in modern days brought so many of these little objects withur cognisance. I think, however, it must be admitted that such discoveries have a totally

fferent importance in the history of science from that which belongs to the peerlesschievement of Herschel. In the first place, it must be observed that the minor planets nowought to light are so minute that if a score of them were rolled to together into one lumpould not be one-thousandth part of the size of the grand planet discovered by Herschel. T nevertheless, not the most important point. What marks Herschel's achievement as one e great epochs in the history of astronomy is the fact that the detection of Uranus was th

ery first recorded occasion of the discovery of any planet whatever.

or uncounted ages those who watched the skies had been aware of the existence of the fi

d planets-Jupiter, Mercury, Saturn, Venus, and Mars. It never seems to have occurred to the ancient philosophers that there could be other similar objects as yet undetected over

nd above the well-known five. Great then was the astonishment of the scientific world whee Bath organist announced his discovery that the five planets which had been known from antiquity must now admit the company of a sixth. And this sixth planet was, indeed, wor

n every ground to be received into the ranks of the five glorious bodies of antiquity. It waso doubt, not so large as Saturn, it was certainly very much less than Jupiter; on the otherand, the new body was very much larger than Mercury, than Venus, or than Mars, and thearth itself seemed quite an insignificant object in comparison with this newly added memb

the Solar System. In one respect, too, Herschel's new planet was a much more imposingbject than any one of the older bodies; it swept around the sun in a majestic orbit, farutside that of Saturn, which had previously been regarded as the boundary of the Solarystem, and its stately progress required a period of not less than eighty-one years.

ng George the Third, hearing of the achievements of the Hanoverian musician, felt muchterest in his discovery, and accordingly Herschel was bidden to come to Windsor, and toing with him the famous telescope, in order to exhibit the new planet to the King, and to s Majesty all about it. The result of the interview was to give Herschel the opportunity for

hich he had so long wished, of being able to devote himself exclusively to science for thest of his life.

he King took so great a fancy to the astronomer that he first, as I have already mentioneduly pardoned his desertion from the army, some twenty-five years previously. As a furtherark of his favour the King proposed to confer on Herschel the title of his Majesty's owntronomer, to assign to him a residence near Windsor, to provide him with a salary, and tornish such funds as might be required for the erection of great telescopes, and for the

onduct of that mighty scheme of celestial observation on which Herschel was so eager to




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nter. Herschel's capacity for work would have been much impaired if he had been deprivee aid of his admirable sister, and to her, therefore, the King also assigned a salary, and sas installed as Herschel's assistant in his new post.

ith his usually impulsive determination, Herschel immediately cut himself free from all hisusical avocations at Bath, and at once entered on the task of making and erecting the grelescopes at Windsor. There, for more than thirty years, he and his faithful sister prosecute

th unremitting ardour their nightly scrutiny of the sky. Paper after paper was sent to theoyal Society, describing the hundreds, indeed the thousands, of objects such as double staebulae and clusters, which were first revealed to human gaze during those midnight vigilse end of his life he still continued at every possible opportunity to devote himself to that

eloved pursuit in which he had such unparalleled success. No single discovery of Herschel'ter years was, however, of the same momentous description as that which first brought hfame.

erschel married when considerably advanced in life and he lived to enjoy the indescribable

easure of finding that his only son, afterwards Sir John Herschel, was treading worthily inotsteps, and attaining renown as an astronomical observer, second only to that of his fathhe elder Herschel died in 1822, and his illustrious sister Caroline then returned to Hanoverhere she lived for many years to receive the respect and attention which were so justly hehe died at a very advanced age in 1848.

LAPLACE.

he author of the "Mecanique Celeste" was born at Beaumont-en- Auge, near Honfleur, in

749, just thirteen years later than his renowned friend Lagrange. His father was a farmer, ppears to have been in a position to provide a good education for a son who seemedomising. Considering the unorthodoxy in religious matters which is generally said to have

haracterized Laplace in later years, it is interesting to note that when he was a boy thebject which first claimed his attention was theology. He was, however, soon introduced toe study of mathematics, in which he presently became so proficient, that while he was st

o more than eighteen years old, he obtained employment as a mathematical teacher in hisative town.

esiring wider opportunities for study and for the acquisition of fame than could be obtainethe narrow associations of provincial life, young Laplace started for Paris, being providedth letters of introduction to D'Alembert, who then occupied the most prominent position aathematician in France, if not in the whole of Europe. D'Alembert's fame was indeed soilliant that Catherine the Great wrote to ask him to undertake the education of her Son, aomised the splendid income of a hundred thousand francs. He preferred, however, a quiee of research in Paris, although there was but a modest salary attached to his office. Thehilosopher accordingly declined the alluring offer to go to Russia, even though Catherine




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rote again to say: "I know that your refusal arises from your desire to cultivate your studind your friendships in quiet. But this is of no consequence: bring all your friends with you,nd I promise you that both you and they shall have every accommodation in my power."ith equal firmness the illustrious mathematician resisted the manifold attractions with whiederick the Great sought to induce him, to take up his residence at Berlin. In reading of ese invitations we cannot but be struck at the extraordinary respect which was then paid ientific distinction. It must be remembered that the discoveries of such a man as D'Alemb

ere utterly incapable of being appreciated except by those who possessed a high degree oathematical culture. We nevertheless find the potentates of Russia and Prussia entreating

nd, as it happens, vainly entreating, the most distinguished mathematician in France toccept the positions that they were proud to offer him.

was to D'Alembert, the profound mathematician, that young Laplace, the son of the counrmer, presented his letters of introduction. But those letters seem to have elicited no replyhereupon Laplace wrote to D'Alembert submitting a discussion on some point in Dynamicshis letter instantly produced the desired effect. D'Alembert thought that such mathematica

lent as the young man displayed was in itself the best of introductions to his favour. It coot be overlooked, and accordingly he invited Laplace to come and see him. Laplace, of ourse, presented himself, and ere long D'Alembert obtained for the rising philosopher aofessorship of mathematics in the Military School in Paris. This gave the brilliant youngathematician the opening for which he sought, and he quickly availed himself of it.

aplace was twenty-three years old when his first memoir on a profound mathematical subjppeared in the Memoirs of the Academy at Turin. From this time onwards we find himublishing one memoir after another in which he attacks, and in many cases successfully

anquishes, profound difficulties in the application of the Newtonian theory of gravitation toe explanation of the solar system. Like his great contemporary Lagrange, he loftilytempted problems which demanded consummate analytical skill for their solution. Thetention of the scientific world thus became riveted on the splendid discoveries which

manated from these two men, each gifted with extraordinary genius.

aplace's most famous work is, of course, the "Mecanique Celeste," in which he essayed aomprehensive attempt to carry out the principles which Newton had laid down, into mucheater detail than Newton had found practicable. The fact was that Newton had not only t

onstruct the theory of gravitation, but he had to invent the mathematical tools, so to speay which his theory could be applied to the explanation of the movements of the heavenlyodies. In the course of the century which had elapsed between the time of Newton and thme of Laplace, mathematics had been extensively developed. In particular, that potentstrument called the infinitesimal calculus, which Newton had invented for the investigationature, had become so far perfected that Laplace, when he attempted to unravel theovements of the heavenly bodies, found himself provided with a calculus far more efficienan that which had been available to Newton. The purely geometrical methods which New

mployed, though they are admirably adapted for demonstrating in a general way the




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me direction. Nor did it escape his attention that the sun itself rotated on its axis in theme sense. His philosophical mind was led to reflect that such a remarkable unanimity in trection of the movements in the solar system demanded some special explanation. It wouave been in the highest degree improbable that there should have been this unanimity unere had been some physical reason to account for it. To appreciate the argument let us fi

oncentrate our attention on three particular bodies, namely the earth, the sun, and theoon. First the earth revolves around the sun in a certain direction, and the earth also rota

n its axis. The direction in which the earth turns in accordance with this latter movementight have been that in which it revolves around the sun, or it might of course have beenpposite thereto. As a matter of fact the two agree. The moon in its monthly revolutionound the earth follows also the same direction, and our satellite rotates on its axis in theme period as its monthly revolution, but in doing so is again observing this same law. We

ave therefore in the earth and moon four movements, all taking place in the same directiond this is also identical with that in which the sun rotates once every twenty-five days. Succoincidence would be very unlikely unless there were some physical reason for it. Just asnlikely would it be that in tossing a coin five heads or five tails should follow each other

onsecutively. If we toss a coin five times the chances that it will turn up all heads or all taiut a small one. The probability of such an event is only one-sixteenth.

here are, however, in the solar system many other bodies besides the three just mentionehich are animated by this common movement. Among them are, of course, the greatanets, Jupiter, Saturn, Mars, Venus, and Mercury, and the satellites which attend on theseanets. All these planets rotate on their axes in the same direction as they revolve around n, and all their satellites revolve also in the same way. Confining our attention merely to

arth, the sun, and the five great planets with which Laplace was acquainted, we have no

wer than six motions of revolution and seven motions of rotation, for in the latter we inclue rotation of the sun. We have also sixteen satellites of the planets mentioned whosevolutions round their primaries are in the same direction. The rotation of the moon on its

xis may also be reckoned, but as to the rotations of the satellites of the other planets wennot speak with any confidence, as they are too far off to be observed with the necessary

ccuracy. We have thus thirty circular movements in the solar system connected with the snd moon and those great planets than which no others were known in the days of Laplacehe significant fact is that all these thirty movements take place in the same direction. Thatis should be the case without some physical reason would be just as unlikely as that in

ssing a coin thirty times it should turn up all heads or all tails every time without exceptio

e can express the argument numerically. Calculation proves that such an event would notenerally happen oftener than once out of five hundred millions of trials. To a philosopher oaplace's penetration, who had made a special study of the theory of probabilities, it seemeell-nigh inconceivable that there should have been such unanimity in the celestialovements, unless there had been some adequate reason to account for it. We might, inde

dd that if we were to include all the objects which are now known to belong to the solarstem, the argument from probability might be enormously increased in strength. To Lapla




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e argument appeared so conclusive that he sought for some physical cause of themarkable phenomenon which the solar system presented. Thus it was that the famousebular Hypothesis took its rise. Laplace devised a scheme for the origin of the sun and theanetary system, in which it would be a necessary consequence that all the movementsould take place in the same direction as they are actually observed to do.

et us suppose that in the beginning there was a gigantic mass of nebulous material, so hig

eated that the iron and other substances which now enter into the composition of the eartnd planets were then suspended in a state of vapour. There is nothing unreasonable in susupposition indeed, we know as a matter of fact that there are thousands of such nebulaee discerned at present through our telescopes. It would be extremely unlikely that any objould exist without possessing some motion of rotation; we may in fact assert that for rotat

be entirety absent from the great primeval nebula would be almost infinitely improbable.ges rolled on, the nebula gradually dispersed away by radiation its original stores of heat,nd, in accordance with well-known physical principles, the materials of which it was formeould tend to coalesce. The greater part of those materials would become concentrated in

ighty mass surrounded by outlying uncondensed vapours. There would, however, also begions throughout the extent of the nebula, in which subsidiary centres of condensationould be found. In its long course of cooling, the nebula would, therefore, tend ultimately trm a mighty central body with a number of smaller bodies disposed around it. As the nebas initially endowed with a movement of rotation, the central mass into which it had chiefondensed would also revolve, and the subsidiary bodies would be animated by movementsvolution around the central body. These movements would be all pursued in one commonrection, and it follows, from well-known mechanical principles, that each of the subsidiaryasses, besides participating in the general revolution around the central body, would also

ossess a rotation around its axis, which must likewise be performed in the same direction.ound the subsidiary bodies other objects still smaller would be formed, just as theyemselves were formed relatively to the great central mass.

s the ages sped by, and the heat of these bodies became gradually dissipated, the variousbjects would coalesce, first into molten liquid masses, and thence, at a further stage of ooling, they would assume the appearance of solid masses, thus producing the planetaryodies such as we now know them. The great central mass, on account of its preponderatinmensions, would still retain, for further uncounted ages, a large quantity of its primeval h

nd would thus display the splendours of a glowing sun. In this way Laplace was able toccount for the remarkable phenomena presented in the movements of the bodies of the sostem. There are many other points also in which the nebular theory is known to tally withe facts of observation. In fact, each advance in science only seems to make it more certaat the Nebular Hypothesis substantially represents the way in which our solar system hasown to its present form.

ot satisfied with a career which should be merely scientific, Laplace sought to connectmself with public affairs. Napoleon appreciated his genius, and desired to enlist him in the




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ate on which the celebration of the tercentenary of the University was held happens tooincide with the centenary of the first visitation of the observatory. The visitors on the firstccasion were A. Murray, Matthew Young, George Hall, and John Barrett. They record thatey find the buildings, books and instruments in good condition; but the chief feature in thport, as well as in many which followed it, related to a circumstance to which we have no

et referred.

the original equipment of the observatory, Ussher, with the natural ambition of a foundeesired to place in it a telescope of more magnificent proportions than could be foundnywhere else. The Board gave a spirited support to this enterprise, and negotiations werentered into with the most eminent instrument- maker of those days. This was Jesse Rams735-1800), famous as the improver of the sextant, as the constructor of the great theodo

sed by General Roy in the English Survey, and as the inventor of the dividing engine foraduating astronomical instruments. Ramsden had built for Sir George Schuckburgh thergest and most perfect equatorial ever attempted. He had constructed mural quadrants foadua and Verona, which elicited the wonder of astronomers when Dr. Maskelyne declared

ould detect no error in their graduation so large as two seconds and a half. But Ramsdenaintained that even better results would be obtained by superseding the entire quadrant be circle. He obtained the means of testing this prediction when he completed a superb cirr Palermo of five feet diameter. Finding his anticipations were realised, he desired to apple same principles on a still grander scale. Ramsden was in this mood when he met with Dssher. The enthusiasm of the astronomer and the instrument-maker communicated itself te Board, and a tremendous circle, to be ten feet in diameter, was forthwith projected.

ojected, but never carried out. After Ramsden had to some extent completed a 10-foot

rcle, he found such difficulties that he tried a 9-foot, and this again he discarded for an 8-ot, which was ultimately accomplished, though not entirely by himself. Notwithstanding th

ontraction from the vast proportions originally designed, the completed instrument must ste regarded as a colossal piece of astronomical workmanship. Even at this day I do not knoat any other observatory can show a circle eight feet in diameter graduated all round.

think it is Professor Piazzi Smith who tells us how grateful he was to find a large telescopead ordered finished by the opticians on the very day they had promised it. The day waserfectly correct; it was only the year that was wrong. A somewhat remarkable experience

is direction is chronicled by the early reports of the visitors to Dunsink Observatory. I cannd the date on which the great circle was ordered from Ramsden, but it is fixed withfficient precision by an allusion in Ussher's paper to the Royal Irish Academy, which showat by the 13th June, 1785, the order had been given, but that the abandonment of the 10ot scale had not then been contemplated. It was reasonable that the board should allowamsden ample time for the completion of a work at once so elaborate and so novel. It couot have been finished in a year, nor would there have been much reason for complaint if taker had found he required two or even three years more.




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even years gone, and still no telescope, was the condition in which the Board found mattetheir first visitation in 1792. They had, however, assurances from Ramsden that the

strument would be completed within the year; but, alas for such promises, another sevenears rolled on, and in 1799 the place for the great circle was still vacant at Dunsink. Ramsad fallen into bad health, and the Board considerately directed that "inquiries should beade." Next year there was still no progress, so the Board were roused to threaten Ramsdeth a suit at law; but the menace was never executed, for the malady of the great optician

ew worse, and he died that year.

fairs had now assumed a critical aspect, for the college had advanced much money toamsden during these fifteen years, and the instrument was still unfinished. An appeal wasade by the Provost to Dr. Maskelyne, the Astronomer Royal of England, for his advice andndly offices in this emergency. Maskelyne responds-- in terms calculated to allay the anxiethe Bursar--"Mr. Ramsden has left property behind him, and the College can be in no

anger of losing both their money and the instrument." The business of Ramsden was thenndertaken by Berge, who proceeded to finish the circle quite as deliberately as his

edecessor. After four years Berge promised the instrument in the following August, but itot come. Two years later (1806) the professor complains that he can get no answer fromerge. In 1807, it is stated that Berge will send the telescope in a month. He did not; but ine next year (1808), about twenty-three years after the great circle was ordered, it wasected at Dunsink, where it is still to be seen.

he following circumstances have been authenticated by the signatures of Provosts, Proctoursars, and other College dignitaries:--In 1793 the Board ordered two of the clocks at thebservatory to be sent to Mr. Crosthwaite for repairs. Seven years later, in 1800, Mr.

rosthwaite was asked if the clocks were ready. This impatience was clearly unreasonable, ven in four more years, 1804, we find the two clocks were still in hand. Two years later, in806, the Board determined to take vigorous action by asking the Bursar to call uponrosthwaite. This evidently produced some effect, for in the following year, 1807, theofessor had no doubt that the clocks would be speedily returned. After eight years more,

815, one of the clocks was still being repaired, and so it was in 1816, which is the last rece have of these interesting timepieces. Astronomers are, however, accustomed to deal wich stupendous periods in their calculations, that even the time taken to repair a clock see

ut small in comparison.

he long tenure of the chair of Astronomy by Brinkley is divided into two nearly equal perioy the year in which the great circle was erected. Brinkley was eighteen years waiting for hlescope, and he had eighteen years more in which to use it. During the first of these perioinkley devoted himself to mathematical research; during the latter he became a celebratetronomer. Brinkley's mathematical labours procured for their author some reputation as aathematician. They appear to be works of considerable mathematical elegance, but notdicating any great power of original thought. Perhaps it has been prejudicial to Brinkley'sme in this direction, that he was immediately followed in his chair by so mighty a genius a




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illiam Rowan Hamilton.

ter the great circle had been at last erected, Brinkley was able to begin his astronomicalork in earnest. Nor was there much time to lose. He was already forty-five years old, a yeder than was Herschel when he commenced his immortal career at Slough. Stimulated bye consciousness of having the command of an instrument of unique perfection, Brinkleyftily attempted the very highest class of astronomical research. He resolved to measure

new with his own eye and with his own hand the constants of aberration and of nutation. so strove to solve that great problem of the universe, the discovery of the distance of a fiar.

hese were noble problems, and they were nobly attacked. But to appraise with justice thisork of Brinkley, done seventy years ago, we must not apply to it the same criterion as weould think right to apply to similar work were it done now. We do not any longer useinkley's constant of aberration, nor do we now think that Brinkley's determinations of thear distances were reliable. But, nevertheless, his investigations exercised a marked influen

n the progress of science; they stimulated the study of the principles on which exacteasurements were to be conducted.

inkley had another profession in addition to that of an astronomer. He was a divine. Whean endeavours to pursue two distinct occupations concurrently, it will be equally easy to

xplain why his career should be successful, or why it should be the reverse. If he succeedse will, of course, exemplify the wisdom of having two strings to his bow. Should he fail, it course, because he has attempted to sit on two stools at once. In Brinkley's case, his twoofessions must be likened to the two strings rather than to the two stools. It is true that h

actical experience of his clerical life was very slender. He had made no attempt to combine routine of a parish with his labours in the observatory. Nor do we associate a special

minence in any department of religious work with his name. If, however, we are to measuinkley's merits as a divine by the ecclesiastical preferment which he received, his serviceseology must have rivalled his services to astronomy. Having been raised step by step in thurch, he was at last appointed to the See of Cloyne, in 1826, as the successor of Bishoperkeley.

ow, though it was permissible for the Archdeacon to be also the Andrews Professor, yet

hen the Archdeacon became a Bishop, it was understood that he should transfer hissidence from the observatory to the palace. The chair of Astronomy accordingly became

acant. Brinkley's subsequent career seems to have been devoted entirely to ecclesiasticalatters, and for the last ten years of his life he did not contribute a paper to any scientific

ociety. Arago, after a characteristic lament that Brinkley should have forsaken the pursuit oience for the temporal and spiritual attractions of a bishopric, pays a tribute to the

onscientiousness of the quondam astronomer, who would not even allow a telescope to beought into the palace lest his mind should be distracted from his sacred duties.




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e Socratic method, by putting another question: 'And what do you yourself suppose is thedest of all things?' The boy was not successful in his answers, thereon the old astronomerok up a small stone from the garden walk: "There, my child, there is the oldest of all theings that I certainly know.' On another occasion his father is said to have asked the boy,

What sort of things, do you think, are most alike?' The delicate, blue-eyed boy, after a shoause, replied, 'The leaves of the same tree are most like each other.' 'Gather, then, a handleaves of that tree,' rejoined the philosopher, 'and choose two that are alike.' The boy

iled; but he hid the lesson in his heart, and his thoughts were revealed after many days.hese incidents may be trifles; nor should we record them here had not John Herschelmself, though singularly reticent about his personal emotions, recorded them as havingade a strong impression on his mind. Beyond all doubt we can trace therein, first, that gra

nd grouping of many things in one, implied in the stone as the oldest of things; and,condly, that fine and subtle discrimination of each thing out of many like things as formine main features which characterized the habit of our venerated friend's philosophy."

hn Herschel entered St. John's College, Cambridge, when he was seventeen years of age

s university career abundantly fulfilled his father's eager desire, that his only son shouldevelop a capacity for the pursuit of science. After obtaining many lesser distinctions, henally came out as Senior Wrangler in 1813. It was, indeed, a notable year in theathematical annals of the University. Second on that list, in which Herschel's name was fir

ppeared that of the illustrious Peacock, afterwards Dean of Ely, who remained throughoutne of Herschel's most intimate friends.

most immediately after taking his degree, Herschel gave evidence of possessing a specialptitude for original scientific investigation. He sent to the Royal Society a mathematical pa

hich was published in the PHILOSOPHICAL TRANSACTIONS. Doubtless the splendour thattached to the name he bore assisted him in procuring early recognition of his own greatowers. Certain it is that he was made a Fellow of the Royal Society at the unprecedentedlyarly age of twenty-one. Even after this remarkable encouragement to adopt a scientificreer as the business of his life, it does not seem that John Herschel at first contemplated

evoting himself exclusively to science. He commenced to prepare for the profession of theaw by entering as a student at the Middle Temple, and reading with a practising barrister.

ut a lawyer John Herschel was not destined to become. Circumstances brought him into

sociation with some leading scientific men. He presently discovered that his inclinationsnded more and more in the direction of purely scientific pursuits. Thus it came to pass thae original intention as to the calling which he should follow was gradually abandoned.

ortunately for science Herschel found its pursuit so attractive that he was led, as his fathead been before him, to give up his whole life to the advancement of knowledge. Nor was innatural that a Senior Wrangler, who had once tasted the delights of mathematical researould have been tempted to devote much time to this fascinating pursuit. By the time Joh

erschel was twenty-nine he had published so much mathematical work, and his researcheere considered to possess so much merit, that the Royal Society awarded him the Copley




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edal, which was the highest distinction it was capable of conferring.

the death of his father in 1822, John Herschel, with his tastes already formed for a scienreer, found himself in the possession of ample means. To him also passed all his father'seat telescopes and apparatus. These material aids, together with a dutiful sense of filial

bligation, decided him to make practical astronomy the main work of his life. He decided tontinue to its completion that great survey of the heavens which had already been

augurated, and, indeed, to a large extent accomplished, by his father.

he first systematic piece of practical astronomical work which John Herschel undertook waonnected with the measurement of what are known as "Double Stars." It should be observat there are in the heavens a number of instances in which two stars are seen in very closociation. In the case of those objects to which the expression "Double Stars" is generally

pplied, the two luminous points are so close together that even though they might each beuite bright enough to be visible to the unaided eye, yet their proximity is such that theynnot be distinguished as two separate objects without optical aid. The two stars seem fus

gether into one. In the telescope, however, the bodies may be discerned separately, thouey are frequently so close together that it taxes the utmost power of the instrument todicate the division between them.

he appearance presented by a double star might arise from the circumstance that the twoars, though really separated from each other by prodigious distances, happened to lie neathe same line of vision, as seen from our point of view. No doubt, many of the so-called

ouble stars could be accounted for on this supposition. Indeed, in the early days when butw double stars were known, and when telescopes were not powerful enough to exhibit th

umerous close doubles which have since been brought to light, there seems to have been ndency to regard all double stars as merely such perspective effects. It was not at firstggested that there could be any physical connection between the components of each pa

he appearance presented was regarded as merely due to the circumstance that the lineining the two bodies happened to pass near the earth.

the early part of his career, Sir William Herschel seems to have entertained the view theenerally held by other astronomers with regard to the nature of these stellar pairs. The grbserver thought that the double stars could therefore be made to afford a means of solvin

at problem in which so many of the observers of the skies had been engaged, namely, thetermination of the distances of the stars from the earth. Herschel saw that the displacemthe earth in its annual movement round the sun would produce an apparent shift in the

ace of the nearer of the two stars relatively to the other, supposed to be much more remothis shift could be measured, then the distance of the nearer of the stars could be estimath some degree of precision.

s has not unfrequently happened in the history of science, an effect was perceived of a ve




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as rewarded by the accumulation of so great a mass of careful measurements that whenublished, they formed quite a volume in the "Philosophical Transactions." The value andccuracy of the work, when estimated by standards which form proper criteria for that periouniversally recognised. It greatly promoted the progress of sidereal astronomy, and the

uthors were in consequence awarded medals from the Royal Society, and the Royalstronomical Society, as well as similar testimonials from various foreign institutions.

his work must, however, be regarded as merely introductory to the main labours of Johnerschel's life. His father devoted the greater part of his years as an observer to what helled his "sweeps" of the heavens. The great reflecting telescope, twenty feet long, wasoved slowly up and down through an arc of about two degrees towards and from the polehile the celestial panorama passed slowly in the course of the diurnal motion before theeenly watching eye of the astronomer. Whenever a double star traversed the field Herscheescribed it to his sister Caroline, who, as we have already mentioned, was his invariablesistant in his midnight watches. When a nebula appeared, then he estimated its size and ightness, he noticed whether it had a nucleus, or whether it had stars disposed in any

gnificant manner with regard to it. He also dictated any other circumstance which he deemorthy of record. These observations were duly committed to writing by the same faithful adefatigable scribe, whose business it also was to take a memorandum of the exact positiothe object as indicated by a dial placed in front of her desk, and connected with thelescope.

hn Herschel undertook the important task of re-observing the various double stars andebulae which had been discovered during these memorable vigils. The son, however, lackne inestimable advantage which had been possessed by the father. John Herschel had no

sistant to discharge all those duties which Caroline had so efficiently accomplished. He haerefore, to modify the system of sweeping previously adopted in order to enable all the w

oth of observing and of recording to be done by himself. This, in many ways, was a greatawback to the work of the younger astronomer. The division of labour between the obser

nd the scribe enables a greatly increased quantity of work to be got through. It is alsostinctly disadvantageous to an observer to have to use his eye at the telescope directly afe has been employing it for reading the graduations on a circle, by the light of a lamp, or ntering memoranda in a note book. Nebulae, especially, are often so excessively faint thatey can only be properly observed by an eye which is in that highly sensitive condition wh

obtained by long continuance in darkness. The frequent withdrawal of the eye from the deld of the telescope, and the application of it to reading by artificial light, is very prejudicias use for the more delicate purpose. John Herschel, no doubt, availed himself of everyecaution to mitigate the ill effects of this inconvenience as much as possible, but it must

ave told upon his labours as compared with those of his father.

ut nevertheless John Herschel did great work during his "sweeps." He was specially particnote all the double stars which presented themselves to his observation. Of course some

tle discretion must be allowed in deciding as to what degree of proximity in adjacent stars




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he first few lines of the eulogium just quoted allude to Herschel's absence from England. Tas not merely an episode of interest in the career of Herschel, it was the occasion of one e greatest scientific expeditions in the whole history of astronomy.

erschel had, as we have seen, undertaken a revision of his father's "sweeps" for new objethose skies which are visible from our latitudes in the northern hemisphere. He had well-

gh completed this task. Zone by zone the whole of the heavens which could be observedom Windsor had passed under his review. He had added hundreds to the list of nebulaescovered by his father. He had announced thousands of double stars. At last, however, theat survey was accomplished. The contents of the northern hemisphere, so far at least asey could be disclosed by his telescope of twenty feet focal length, had been revealed.

ut Herschel felt that this mighty task had to be supplemented by another of almost equaloportions, before it could be said that the twenty-foot telescope had done its work. It wa

nly the northern half of the celestial sphere which had been fully explored. The southern h

as almost virgin territory, for no other astronomer was possessed of a telescope of suchower as those which the Herschels had used. It is true, of course, that as a certain margine southern hemisphere was visible from these latitudes, it had been more or less scrutiniz

y observers in northern skies. And the glimpses which had thus been obtained of the celesbjects in the southern sky, were such as to make an eager astronomer long for a closercquaintance with the celestial wonders of the south. The most glorious object in the sidereeavens, the Great Nebula in Orion, lies indeed in that southern hemisphere to which theounger Herschel's attention now became directed. It fortunately happens, however, forotaries of astronomy all the world over, that Nature has kindly placed her most astounding

bject, the great Nebula in Orion, in such a favoured position, near the equator, that from aonsiderable range of latitudes, both north and south, the wonders of the Nebula can bexplored. There are grounds for thinking that the southern heavens contain noteworthybjects which, on the whole, are nearer to the solar system than are the noteworthy objecte northern skies. The nearest star whose distance is known, Alpha Centauri, lies in the

outhern hemisphere, and so also does the most splendid cluster of stars.

fluenced by the desire to examine these objects, Sir John Herschel determined to take hiseat telescope to a station in the southern hemisphere, and thus complete his survey of th

dereal heavens. The latitude of the Cape of Good Hope is such that a suitable site could bere found for his purpose. The purity of the skies in South Africa promised to provide for tronomer those clear nights which his delicate task of surveying the nebulae would requir

n November 13, 1833, Sir John Herschel, who had by this time received the honour of nighthood from William IV., sailed from Portsmouth for the Cape of Good Hope, taking witm his gigantic instruments. After a voyage of two months, which was considered to be a fassage in those days, he landed in Table Bay, and having duly reconnoitred various localite decided to place his observatory at a place called Feldhausen, about six miles from Cape




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own, near the base of the Table Mountain. A commodious residence was there available, ait he settled with his family. A temporary building was erected to contain the equatorial, e great twenty-foot telescope was accommodated with no more shelter than is provided be open canopy of heaven.

s in his earlier researches at home, the attention of the great astronomer at the Cape of ood Hope was chiefly directed to the measurement of the relative positions and distances

part of the double stars, and to the close examination of the nebulae. In the delineation oe form of these latter objects Herschel found ample employment for his skilful pencil. Manthe drawings he has made of the celestial wonders in the southern sky are admirable

xamples of celestial portraiture.

he number of the nebulae and of those kindred objects, the star clusters, which Herscheludied in the southern heavens, during four years of delightful labour, amount in all to oneousand seven hundred and seven. His notes on their appearance, and the determinationseir positions, as well as his measurements of double stars, and much other valuable

tronomical research, were published in a splendid volume, brought out at the cost of theuke of Northumberland. This is, indeed, a monumental work, full of interesting andstructive reading for any one who has a taste for astronomy.

erschel had the good fortune to be at the Cape on the occasion of the periodical return ofalley's great comet in 1833. To the study of this body he gave assiduous attention, and thcords of his observations form one of the most interesting chapters in that remarkable

olume to which we have just referred.

arly in 1838 Sir John Herschel returned to England. He had made many friends at the Capho deeply sympathised with his self- imposed labours while he was resident among them.hey desired to preserve the recollection of this visit, which would always, they considered,source of gratification in the colony. Accordingly, a number of scientific friends in that pare world raised a monument with a suitable inscription, on the spot which had been occup

y the great twenty-foot reflector at Feldhausen.

s return to England after five years of absence was naturally an occasion for much rejoicimong the lovers of astronomy. He was entertained at a memorable banquet, and the Que

her coronation, made him a baronet. His famous aunt Caroline, at that time aged eighty,as still in the enjoyment of her faculties, and was able to estimate at its true value therther lustre which was added to the name she bore. But there is reason to believe that hetisfaction was not quite unmixed with other feelings. With whatever favour she might reg

er nephew, he was still not the brother to whom her life had been devoted. So jealous wais vigorous old lady of the fame of the great brother William, that she could hardly hear w

atience of the achievements of any other astronomer, and this failing existed in some degven when that other astronomer happened to be her illustrious nephew.




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ith Sir John Herschel's survey of the Southern Hemisphere it may be said that his career an observing astronomer came to a close. He did not again engage in any systematiclescopic research. But it must not be inferred from this statement that he desisted from

ctive astronomical work. It has been well observed that Sir John Herschel was perhaps thenly astronomer who has studied with success, and advanced by original research, everyepartment of the great science with which his name is associated. It was to some otheranches of astronomy besides those concerned with looking through telescopes, that the rthe astronomer's life was to be devoted.

o the general student Sir John Herschel is best known by the volume which he publishednder the title of "Outlines of Astronomy." This is, indeed, a masterly work, in which theharacteristic difficulties of the subject are resolutely faced and expounded with as muchmplicity as their nature will admit. As a literary effort this work is admirable, both on accoits picturesque language and the ennobling conceptions of the universe which it unfolds.

he student who desires to become acquainted with those recondite departments of tronomy, in which the effects of the disturbing action of one planet upon the motions of

nother planet are considered, will turn to the chapters in Herschel's famous work on thebject. There he will find this complex matter elucidated, without resort to difficultathematics. Edition after edition of this valuable work has appeared, and though the

dvances of modern astronomy have left it somewhat out of date in certain departments, ye expositions it contains of the fundamental parts of the science still remain unrivalled.

nother great work which Sir John undertook after his return from the Cape, was a naturalmax to those labours on which his father and he had been occupied for so many years. W

ave already explained how the work of both these observers had been mainly devoted to tudy of the nebulae and the star clusters. The results of their discoveries had beennnounced to the world in numerous isolated memoirs. The disjointed nature of theseublications made their use very inconvenient. But still it was necessary for those who desir

study the marvellous objects discovered by the Herschels, to have frequent recourse to tiginal works. To incorporate all the several observations of nebular into one great systemtalogue, seemed, therefore, to be an indispensable condition of progress in this branch o

nowledge. No one could have been so fitted for this task as Sir John Herschel. He, therefotacked and carried through the great undertaking. Thus at last a grand catalogue of nebu

nd clusters was produced. Never before was there so majestic an inventory. If we remembat each of the nebulae is an object so vast, that the whole of the solar system would form

n inconsiderable speck by comparison, what are we to think of a collection in which thesebjects are enumerated in thousands? In this great catalogue we find arranged in systematder all the nebulae and all the clusters which had been revealed by the diligence of theerschels, father and son, in the Northern Hemisphere, and of the son alone in the Southeremisphere. Nor should we omit to mention that the labours of other astronomers werekewise incorporated. It was unavoidable that the descriptions given to each of the objectsould be very slight. Abbreviations are used, which indicate that a nebula is bright, or very




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ight, or extremely bright, or faint, or very faint, or extremely faint. Such phrases haveertainly but a relative and technical meaning in such a catalogue. The nebulae entered asxtremely bright by the experienced astronomer are only so described by way of contrast toe great majority of these delicate telescopic objects. Most of the nebulae, indeed, are sofficult to see, that they admit of but very slight description. It should be observed thaterschel's catalogue augmented the number of known nebulous objects to more than tenmes that collected into any catalogue which had ever been compiled before the days of

illiam Herschel's observing began. But the study of these objects still advances, and theeat telescopes now in use could probably show at least twice as many of these objects ase contained in the list of Herschel, of which a new and enlarged edition has since beenought out by Dr. Dreyer.

ne of the best illustrations of Sir John Herschel's literary powers is to be found in the addrhich he delivered at the Royal Astronomical Society, on the occasion of presenting a medaMr. Francis Baily, in recognition of his catalogue of stars. The passage I shall here cite

aces in its proper aspect the true merit of the laborious duty involved in such a task as th

hich Mr. Baily had carried through with such success:--

f we ask to what end magnificent establishments are maintained by states and sovereignsrnished with masterpieces of art, and placed under the direction of men of first-rate talen

nd high- minded enthusiasm, sought out for those qualities among the foremost in the ranscience, if we demand QUI BONO? for what good a Bradley has toiled, or a Maskelyne or

azzi has worn out his venerable age in watching, the answer is--not to settle merepeculative points in the doctrine of the universe; not to cater for the pride of man by refinquiries into the remoter mysteries of nature; not to trace the path of our system through

pace, or its history through past and future eternities. These, indeed, are noble ends andhich I am far from any thought of depreciating; the mind swells in their contemplation, antains in their pursuit an expansion and a hardihood which fit it for the boldest enterprise. e direct practical utility of such labours is fully worthy of their speculative grandeur. Thears are the landmarks of the universe; and, amidst the endless and complicated fluctuatioour system, seem placed by its Creator as guides and records, not merely to elevate our

inds by the contemplation of what is vast, but to teach us to direct our actions by referenwhat is immutable in His works. It is, indeed, hardly possible to over-appreciate their valthis point of view. Every well-determined star, from the moment its place is registered,

ecomes to the astronomer, the geographer, the navigator, the surveyor, a point of departhich can never deceive or fail him, the same for ever and in all places, of a delicacy soxtreme as to be a test for every instrument yet invented by man, yet equally adapted for tost ordinary purposes; as available for regulating a town clock as for conducting a navy toe Indies; as effective for mapping down the intricacies of a petty barony as for adjusting

oundaries of Transatlantic empires. When once its place has been thoroughly ascertained refully recorded, the brazen circle with which that useful work was done may moulder, tharble pillar may totter on its base, and the astronomer himself survive only in the gratitudposterity; but the record remains, and transfuses all its own exactness into every




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etermination which takes it for a groundwork, giving to inferior instruments--nay, even tomporary contrivances, and to the observations of a few weeks or days--all the precisiontained originally at the cost of so much time, labour, and expense."

r John Herschel wrote many other works besides those we have mentioned. His "Treatiseeteorology" is, indeed, a standard work on this subject, and numerous articles from theme pen on miscellaneous subjects, which have been collected and reprinted, seemed as a

laxation from his severe scientific studies. Like certain other great mathematicians Herschas also a poet, and he published a translation of the Iliad into blank verse.

his later years Sir John Herschel lived a retired life. For a brief period he had, indeed, beduced to accept the office of Master of the Mint. It was, however, evident that the routinech an occupation was not in accordance with his tastes, and he gladly resigned it, to retuthe seclusion of his study in his beautiful home at Collingwood, in Kent.

s health having gradually failed, he died on the 11th May, 1871, in the seventy-ninth yea

s age.

THE EARL OF ROSSE.

he subject of our present sketch occupies quite a distinct position in scientific history. Unliany others who have risen by their scientific discoveries from obscurity to fame, the greatarl of Rosse was himself born in the purple. His father, who, under the title of Sir Lawrencarsons, had occupied a distinguished position in the Irish Parliament, succeeded on the dehis father to the Earldom which had been recently created. The subject of our present

emoir was, therefore, the third of the Earls of Rosse, and he was born in York on June 17800. Prior to his father's death in 1841, he was known as Lord Oxmantown.

he University education of the illustrious astronomer was begun in Dublin and completed axford. We do not hear in his case of any very remarkable University career. Lord Rosse waowever, a diligent student, and obtained a first-class in mathematics. He always took a greeal of interest in social questions, and was a profound student of political economy. He haat in the House of Commons, as member for King's County, from 1821 to 1834, his ancestate being situated in this part of Ireland.

ord Rosse was endowed by nature with a special taste for mechanical pursuits. Not only he the qualifications of a scientific engineer, but he had the manual dexterity which qualifiem personally to carry out many practical arts. Lord Rosse was, in fact, a skilful mechanic, xperienced founder, and an ingenious optician. His acquaintances were largely among thoho were interested in mechanical pursuits, and it was his delight to visit the works orngineering establishments where refined processes in the arts were being carried on. It haten been stated--and as I have been told by members of his family, truly stated--that on




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arsonstown was famous for its glass:--

We shall conclude this chapter with the glass, there having been several glasshouses set uy the English in Ireland, none in Dublin or other cities, but all of them in the country;mongst which the principal was that of Birre, a market town, otherwise called Parsonstowter one Sir Lawrence Parsons, who, having purchased that lordship, built a goodly housepon it; his son William Parsons having succeeded him in the possession of it; which town i

tuate in Queen's County, about fifty miles (Irish) to the southwest of Dublin, upon theorders of the two provinces of Leinster and Munster; from this place Dublin was furnishedth all sorts of window and drinking glasses, and such other as commonly are in use. One

art of the materials, viz., the sand, they had out of England; the other, to wit the ashes, thade in the place of ash-tree, and used no other. The chiefest difficulty was to get the claye pots to melt the materials in; this they had out of the north."--Chap. XXI., Sect. VIII. "Oe Glass made in Ireland."

rr Castle itself is a noble mansion with reminiscences from the time of Cromwell. It is

rrounded by a moat and a drawbridge of modern construction, and from its windowseautiful views can be had over the varied features of the park. But while the visitors toarsonstown will look with great interest on this residence of an Irish landlord, whose deligwas to dwell in his own country, and among his own people, yet the feature which theyave specially come to observe is not to be found in the castle itself. On an extensive lawn,weeping down from the moat towards the lake, stand two noble masonry walls. They arerreted and clad with ivy, and considerably loftier than any ordinary house. As the visitor

pproaches, he will see between those walls what may at first sight appear to him to be thennel of a steamer lying down horizontally. On closer approach he will find that it is an

mmense wooden tube, sixty feet long, and upwards of six feet in diameter. It is in fact largnough to admit of a tall man entering into it and walking erect right through from one ende other. This is indeed the most gigantic instrument which has ever been constructed for

urpose of exploring the heavens. Closely adjoining the walls between which the great tubewings, is a little building called "The Observatory." In this the smaller instruments areontained, and there are kept the books which are necessary for reference. The observatorso offers shelter to the observers, and provides the bright fire and the cup of warm tea,hich are so acceptable in the occasional intervals of a night's observation passed on the tothe walls with no canopy but the winter sky.

most the first point which would strike the visitor to Lord Rosse's telescope is that thestrument at which he is looking is not only enormously greater than anything of the kind te has ever seen before, but also that it is something of a totally different nature. In andinary telescope he is accustomed to find a tube with lenses of glass at either end, while rge telescopes that we see in our observatories are also in general constructed on the saminciple. At one end there is the object-glass, and at the other end the eye-piece, and of

ourse it is obvious that with an instrument of this construction it is to the lower end of thebe that the eye of the observer must be placed when the telescope is pointed to the skies




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ut in Lord Rosse's telescope you would look in vain for these glasses, and it is not at thewer end of the instrument that you are to take your station when you are going to makeour observations. The astronomer at Parsonstown has rather to avail himself of the ingeniostem of staircases and galleries, by which he is enabled to obtain access to the mouth of eat tube. The colossal telescope which swings between the great walls, like Herschel's grelescope already mentioned, is a reflector, the original invention of which is due of course ewton. The optical work which is accomplished by the lenses in the ordinary telescope is

fected in the type of instrument constructed by Lord Rosse by a reflecting mirror which isaced at the lower end of the vast tube. The mirror in this instrument is made of a metal

onsisting of two parts of copper to one of tin. As we have already seen, this mixture formsoy of a very peculiar nature. The copper and the tin both surrender their distinctive

ualities, and unite to form a material of a very different physical character. The copper isugh and brown, the tin is no doubt silvery in hue, but soft and almost fibrous in texture.hen the two metals are mixed together in the proportions I have stated, the alloy obtainetensely hard and quite brittle being in both these respects utterly unlike either of the twogredients of which it is composed. It does, however, resemble the tin in its whiteness, bu

cquires a lustre far brighter than tin; in fact, this alloy hardly falls short of silver itself in itsilliance when polished.

he first duty that Lord Rosse had to undertake was the construction of this tremendousirror, six feet across, and about four or five inches thick. The dimensions were far in excethose which had been contemplated in any previous attempt of the same kind. Herschel

ad no doubt fashioned one mirror of four feet in diameter, and many others of smallermensions, but the processes which he employed had never been fully published, and it wbvious that, with a large increase in dimensions, great additional difficulties had to be

ncountered. Difficulties began at the very commencement of the process, and werexperienced in one form or another at every subsequent stage. In the first place, the meresting of a great disc of this mixture of tin and copper, weighing something like three or fons, involved very troublesome problems. No doubt a casting of this size, if the material ha

een, for example, iron, would have offered no difficulties beyond those with which everyactical founder is well acquainted, and which he has to encounter daily in the course of hdinary work. But speculum metal is a material of a very intractable description. There is, o

ourse, no practical difficulty in melting the copper, nor in adding the proper proportion of then the copper has been melted. There may be no great difficulty in arranging an

ganization by which several crucibles, filled with the molten material, shall be pouredmultaneously so as to obtain the requisite mass of metal, but from this point the difficultieegin. For speculum metal when cold is excessively brittle, and were the casting permitted ool like an ordinary copper or iron casting, the mirror would inevitably fly into pieces. Lordosse, therefore, found it necessary to anneal the casting with extreme care by allowing it ool very slowly. This was accomplished by drawing the disc of metal as soon as it hadntered into the solid state, though still glowing red, into an annealing oven. There themperature was allowed to subside so gradually, that six weeks elapsed before the mirrorad reached the temperature of the external air. The necessity for extreme precaution in th




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peration of annealing will be manifest if we reflect on one of the accidents which happenen a certain occasion, after the cooling of a great casting had been completed, it was foundn withdrawing the speculum, that it was cracked into two pieces. This mishap was eventuaaced to the fact that one of the walls of the oven had only a single brick in its thickness, aat therefore the heat had escaped more easily through that side than through the otherdes which were built of double thickness. The speculum had, consequently, not cooledniformly, and hence the fracture had resulted. Undeterred, however, by this failure, as we

by not a few other difficulties, into a description of which we cannot now enter, Lord Roseadily adhered to his self-imposed task, and at last succeeded in casting two perfect discshich to commence the tedious processes of grinding and polishing. The magnitude of theperations involved may perhaps be appreciated if I mention that the value of the mereopper and tin entering into the composition of each of the mirrors was about 500 pounds.

no part of his undertaking was Lord Rosse's mechanical ingenuity more taxed than in theevising of the mechanism for carrying out the delicate operations of grinding and polishinge mirrors, whose casting we have just mentioned. In the ordinary operations of the

lescope-maker, such processes had hitherto been generally effected by hand, but, of couch methods became impossible when dealing with mirrors which were as large as a good

zed dinner table, and whose weight was measured by tons. The rough grinding was effecy means of a tool of cast iron about the same size as the mirror, which was moved byitable machinery both backwards and forwards, and round and round, plenty of sand andater being supplied between the mirror and the tool to produce the necessary attrition. Ase process proceeded and as the surface became smooth, emery was used instead of sand

nd when this stage was complete, the grinding tool was removed and the polishing tool wbstituted. The essential part of this was a surface of pitch, which, having been temporari

oftened by heat, was then placed on the mirror, and accepted from the mirror the properrm. Rouge was then introduced as the polishing powder, and the operation was continued

bout nine hours, by which time the great mirror had acquired the appearance of highlyolished silver. When completed, the disc of speculum metal was about six feet across andur inches thick. The depression in the centre was about half an inch. Mounted on a littleuck, the great speculum was then conveyed to the instrument, to be placed in its receptacthe bottom of the tube, the length of which was sixty feet, this being the focal distance oe mirror. Another small reflector was inserted in the great tube sideways, so as to direct t

aze of the observer down upon the great reflector. Thus was completed the most colossal

strument for the exploration of the heavens which the art of man has ever constructed.

was once my privilege to be one of those to whom the illustrious builder of the greatlescope entrusted its use. For two seasons in 1865 and 1866 I had the honour of being Loosse's astronomer. During that time I passed many a fine night in the observer's gallery,xamining different objects in the heavens with the aid of this remarkable instrument. At thme I was there, the objects principally studied were the nebulae, those faint stains of lighthich lie on the background of the sky. Lord Rosse's telescope was specially suited for therutiny of these objects, inasmuch as their delicacy required all the light-grasping power




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hich could be provided.

ne of the greatest discoveries made by Lord Rosse, when his huge instrument was firstrned towards the heavens, consisted in the detection of the spiral character of some of th

ebulous forms. When the extraordinary structure of these objects was first announced, thescovery was received with some degree of incredulity. Other astronomers looked at theme objects, and when they failed to discern--and they frequently did fail to discern--the

piral structure which Lord Rosse had indicated, they drew the conclusion that this spiralructure did not exist. They thought it must be due possibly to some instrumental defect oe imagination of the observer. It was, however, hardly possible for any one who was bothlling and competent to examine into the evidence, to doubt the reality of Lord Rosse'sscoveries. It happens, however, that they have been recently placed beyond all doubt bystimony which it is impossible to gainsay. A witness never influenced by imagination hasow come forward, and the infallible photographic plate has justified Lord Rosse. Among thmarkable discoveries which Dr. Isaac Roberts has recently made in the application of his

hotographic apparatus to the heavens, there is none more striking than that which declare

ot only that the nebulae which Lord Rosse described as spirals, actually do possess theharacter so indicated, but that there are many others of the same description. He has eveought to light the astonishingly interesting fact that there are invisible objects of this clashich have never been seen by human eye, but whose spiral character is visible to theeculiar delicacy of the photographic telescope.

his earlier years, Lord Rosse himself used to be a diligent observer of the heavenly bodieth the great telescope which was completed in the year 1845. But I think that those who

new Lord Rosse well, will agree that it was more the mechanical processes incidental to th

aking of the telescope which engaged his interest than the actual observations with thelescope when it was completed. Indeed one who was well acquainted with him believed Losse's special interest in the great telescope ceased when the last nail had been driven intBut the telescope was never allowed to lie idle, for Lord Rosse always had associated wit

m some ardent young astronomer, whose delight it was to employ to the uttermost thedvantages of his position in exploring the wonders of the sky. Among those who were in tpacity in the early days of the great telescope, I may mention my esteemed friend Dr.hnston Stoney.

uch was the renown of Lord Rosse himself, brought about by his consummate mechanicalenius and his astronomical discoveries, and such the interest which gathered around thearvellous workshops at Birr castle, wherein his monumental exhibitions of optical skill wer

onstructed, that visitors thronged to see him from all parts of the world. His home atarsonstown became one of the most remarkable scientific centres in Great Britain; thithersembled from time to time all the leading men of science in the country, as well as manyustrious foreigners. For many years Lord Rosse filled with marked distinction the exaltedosition of President of the Royal Society, and his advice and experience in practicalechanical matters were always at the disposal of those who sought his assistance. Person




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nd socially Lord Rosse endeared himself to all with whom he came in contact. I rememberne of the attendants telling me that on one occasion he had the misfortune to let fall andeak one of the small mirrors on which Lord Rosse had himself expended many hours of h

ersonal labour. The only remark of his lordship was that "accidents will happen."

he latter years of his life Lord Rosse passed in comparative seclusion; he occasionally wenondon for a brief sojourn during the season, and he occasionally went for a cruise in his

acht; but the greater part of the year he spent at Birr Castle, devoting himself largely to thudy of political and social questions, and rarely going outside the walls of his demesne,xcept to church on Sunday mornings. He died on October 31, 1867.

e was succeeded by his eldest son, the present Earl of Rosse, who has inherited his fatherientific abilities, and done much notable work with the great telescope.

AIRY.

our sketch of the life of Flamsteed, we have referred to the circumstances under which tmous Observatory that crowns Greenwich Hill was founded. We have also had occasion toention that among the illustrious successors of Flamsteed both Halley and Bradley are to cluded. But a remarkable development of Greenwich Observatory from the modesttablishment of early days took place under the direction of the distinguished astronomerhose name is at the head of this chapter. By his labours this temple of science was organsuch a degree of perfection that it has served in many respects as a model for othertronomical establishments in various parts of the world. An excellent account of Airy's car

as been given by Professor H. H. Turner, in the obituary notice published by the Royal

stronomical Society. To this I am indebted for many of the particulars here to be set downoncerning the life of the illustrious Astronomer Royal.

he family from which Airy took his origin came from Kentmere, in Westmoreland. His fatheilliam Airy, belonged to a Lincolnshire branch of the same stock. His mother's maiden namas Ann Biddell, and her family resided at Playford, near Ipswich. William Airy held somemall government post which necessitated an occasional change of residence to different p

the country, and thus it was that his son, George Biddell, came to be born at Alnwick, on7th July, 1801. The boy's education, so far as his school life was concerned was partly

onducted at Hereford and partly at Colchester. He does not, however, seem to have deriveuch benefit from the hours which he passed in the schoolroom. But it was delightful to hispend his holidays on the farm at Playford, where his uncle, Arthur Biddell, showed him

uch kindness. The scenes of his early youth remained dear to Airy throughout his life, andbsequent years he himself owned a house at Playford, to which it was his special delight sort for relaxation during the course of his arduous career. In spite of the defects of hishool training he seems to have manifested such remarkable abilities that his uncle decideenter him in Cambridge University. He accordingly joined Trinity College as a sizar in 181




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nd after a brilliant career in mathematical and physical science he graduated as Seniorrangler in 1823. It may be noted as an exceptional circumstance that, notwithstanding th

emands on his time in studying for his tripos, he was able, after his second term of sidence, to support himself entirely by taking private pupils. In the year after he had takes degree he was elected to a Fellowship at Trinity College.

aving thus gained an independent position, Airy immediately entered upon that career of

ientific work which he prosecuted without intermission almost to the very close of his life.ne of his most interesting researches in these early days is on the subject of Astigmatism,hich defect he had discovered in his own eyes. His investigations led him to suggest a mecorrecting this defect by using a pair of spectacles with lenses so shaped as to counterace derangement which the astigmatic eye impressed upon the rays of light. His researchesis subject were of a very complete character, and the principles he laid down are to theesent day practically employed by oculists in the treatment of this malformation.

n the 7th of December, 1826, Airy was elected to the Lucasian Professorship of Mathemat

the University of Cambridge, the chair which Newton's occupancy had rendered soustrious. His tenure of this office only lasted for two years, when he exchanged it for theumian Professorship. The attraction which led him to desire this change is doubtless to beund in the circumstance that the Plumian Professorship of Astronomy carried with it at thame the appointment of director of the new astronomical observatory, the origin of whichust now be described.

hose most interested in the scientific side of University life decided in 1820 that it would boper to found an astronomical observatory at Cambridge. Donations were accordingly

ought for this purpose, and upwards of 6,000 pounds were contributed by members of theniversity and the public. To this sum 5,000 pounds were added by a grant from theniversity chest, and in 1824 further sums amounting altogether to 7,115 pounds were givey the University for the same object. The regulations as to the administration of the newbservatory placed it under the management of the Plumian Professor, who was to beovided with two assistants. Their duties were to consist in making meridian observations e sun, moon, and the stars, and the observations made each year were to be printed and

ublished. The observatory was also to be used in the educational work of the University, foas arranged that smaller instruments were to be provided by which students could be

structed in the practical art of making astronomical observations.

he building of the Cambridge Astronomical Observatory was completed in 1824, but in 182hen Airy entered on the discharge of his duties as Director, the establishment was still farom completion, in so far as its organisation was concerned. Airy commenced his work sonergetically that in the next year after his appointment he was able to publish the firstolume of "Cambridge Astronomical Observations," notwithstanding that every part of theork, from the making of observations to the revising of the proof-sheets, had to be done bmself.




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may here be remarked that these early volumes of the publications of the Cambridgebservatory contained the first exposition of those systematic methods of astronomical worhich Airy afterwards developed to such a great extent at Greenwich, and which have beenbsequently adopted in many other places. No more profitable instruction for thetronomical beginner can be found than that which can be had by the study of these

olumes, in which the Plumian Professor has laid down with admirable clearness the true

inciples on which meridian work should be conducted.

ry gradually added to the instruments with which the observatory was originally equippedural circle was mounted in 1832, and in the same year a small equatorial was erected bynes. This was made use of by Airy in a well-known series of observations of Jupiter's fourtellite for the determination of the mass of the great planet. His memoir on this subject fu

x pounds the method of finding the weight of a planet from observations of the movemena satellite by which the planet is attended. This is, indeed, a valuable investigation which

udent of astronomy can afford to neglect. The ardour with which Airy devoted himself to

tronomical studies may be gathered from a remarkable report on the progress of astronouring the present century, which he communicated to the British Association at its secondeeting in 1832. In the early years of his life at Cambridge his most famous achievement w

onnected with a research in theoretical astronomy for which consummate mathematicalower was required. We can only give a brief account of the Subject, for to enter into any fetail with regard to it would be quite out of the question.

enus is a planet of about the same size and the same weight as the earth, revolving in anbit which lies within that described by our globe. Venus, consequently, takes less time tha

e earth to accomplish one revolution round the sun, and it happens that the relativeovements of Venus and the earth are so proportioned that in the time in which our earth

ccomplishes eight of her revolutions the other planet will have accomplished almost exactlirteen. It, therefore, follows that if the earth and Venus are in line with the sun at one daen in eight years later both planets will again be found at the same points in their orbits. ose eight years the earth has gone round eight times, and has, therefore, regained itsiginal position, while in the same period Venus has accomplished thirteen completevolutions, and, therefore, this planet also has reached the same spot where it was at firstenus and the earth, of course, attract each other, and in consequence of these mutual

tractions the earth is swayed from the elliptic track which it would otherwise pursue. In lianner Venus is also forced by the attraction of the earth to revolve in a track which deviaom that which it would otherwise follow. Owing to the fact that the sun is of sucheponderating magnitude (being, in fact, upwards of 300,000 times as heavy as either Venthe earth), the disturbances induced in the motion of either planet, in consequence of th

traction of the other, are relatively insignificant to the main controlling agency by which ethe movements is governed. It is, however, possible under certain circumstances that th

sturbing effects produced upon one planet by the other can become so multiplied as tooduce peculiar effects which attain measurable dimensions. Suppose that the periodic tim




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which the earth and Venus revolved had no simple relation to each other, then the pointseir tracks in which the two planets came into line with the sun would be found at differen

arts of the orbits, and consequently the disturbances would to a great extent neutralise eaher, and produce but little appreciable effect. As, however, Venus and the earth come ba

very eight years to nearly the same positions at the same points of their track, anccumulative effect is produced. For the disturbance of one planet upon the other will, of ourse, be greatest when those two planets are nearest, that is, when they lie in line with t

n and on the same side of it. Every eight years a certain part of the orbit of the earth is,erefore, disturbed by the attraction of Venus with peculiar vigour. The consequence is tha

wing to the numerical relation between the movements of the planets to which I haveferred, disturbing effects become appreciable which would otherwise be too small to permrecognition. Airy proposed to himself to compute the effects which Venus would have one movement of the earth in consequence of the circumstance that eight revolutions of the

ne planet required almost the same time as thirteen revolutions of the other. This is aathematical inquiry of the most arduous description, but the Plumian Professor succeededorking it out, and he had, accordingly, the gratification of announcing to the Royal Society

at he had detected the influence which Venus was thus able to assert on the movement our earth around the sun. This remarkable investigation gained for its author the gold medathe Royal Astronomical Society in the year 1832.

consequence Of his numerous discoveries, Airy's scientific fame had become so wellcognised that the Government awarded him a special pension, and in 1835, when Pond,ho was then Astronomer Royal, resigned, Airy was offered the post at Greenwich. There wtruth, no scientific inducement to the Plumian Professor to leave the comparatively easy

ost he held at Cambridge, in which he had ample leisure to devote himself to those

searches which specially interested him, and accept that of the much more arduousbservatory at Greenwich. There were not even pecuniary inducements to make the changeowever, he felt it to be his duty to accede to the request which the Government had madeat he would take up the position which Pond had vacated, and accordingly Airy went toreenwich as Astronomer Royal on October 1st, 1835.

e immediately began with his usual energy to organise the systematic conduct of theusiness of the National Observatory. To realise one of the main characteristics of Airy's greork at Greenwich, it is necessary to explain a point that might not perhaps be understood

thout a little explanation by those who have no practical experience in an observatory. Ine work of an establishment such as Greenwich, an observation almost always consists of easurement of some kind. The observer may, for instance, be making a measurement of me at which a star passes across a spider line stretched through the field of view; on anotccasion his object may be the measurement of an angle which is read off by examiningrough a microscope the lines of division on a graduated circle when the telescope is so

ointed that the star is placed on a certain mark in the field of view. In either case themmediate result of the astronomical observation is a purely numerical one, but it rarelyappens, indeed we may say it never happens, that the immediate numerical result which t




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bservation gives expresses directly the quantity which we are really seeking for. No doubt bservation has been so designed that the quantity we want to find can be obtained from tgures which the measurement gives, but the object sought is not those figures, for there aways a multitude of other influences by which those figures are affected. For example, if abservation were to be perfect, then the telescope with which the observation is made shoue perfectly placed in the exact position which it ought to occupy; this is, however, never thse, for no mechanic can ever construct or adjust a telescope so perfectly as the wants of

tronomer demand. The clock also by which we determine the time of the observation shoe correct, but this is rarely if ever the case. We have to correct our observations for suchrors, that is to say, we have to determine the errors in the positions of our telescopes ande errors in the going of our clocks, and then we have to determine what the observationsould have been had our telescopes been absolutely perfect, and had our clocks beenbsolutely correct. There are also many other matters which have to be attended to in orde

reduce our observations so as to obtain from the figures as yielded to the observer at thlescope the actual quantities which it is his object to determine.

he work of effecting these reductions is generally a very intricate and laborious matter, soat it has not unfrequently happened that while observations have accumulated in an

bservatory, yet the tedious duty of reducing these observations has been allowed to fall inrear. When Airy entered on his duties at Greenwich he found there an enormous mass of

bservations which, though implicitly containing materials of the greatest value totronomers, were, in their unreduced form, entirely unavailable for any useful purpose. Heerefore, devoted himself to coping with the reduction of the observations of hisedecessors. He framed systematic methods by which the reductions were to be effected,

nd he so arranged the work that little more than careful attention to numerical accuracy

ould be required for the conduct of the operations. Encouraged by the Admiralty, for it isnder this department that Greenwich Observatory is placed, the Astronomer Royal employlarge force of computers to deal with the work. BY his energy and admirable organisationanaged to reduce an extremely valuable series of planetary observations, and to publish tsults, which have been of the greatest importance to astronomical investigation.

he Astronomer Royal was a capable, practical engineer as well as an optician, and heesently occupied himself by designing astronomical instruments of improved pattern, whiould replace the antiquated instruments he found in the observatory. In the course of yea

e entire equipment underwent a total transformation. He ordered a great meridian circle,very part of which may be said to have been formed from his own designs. He also designe mounting for a fine equatorial telescope worked by a driving clock, which he had himsevented. Gradually the establishment at Greenwich waxed great under his incessant care. as the custom for the observatory to be inspected every year by a board of visitors, whoshairman was the President of the Royal Society. At each annual visitation, held on the firstaturday in June, the visitors received a report from the Astronomer Royal, in which he setrth the business which had been accomplished during the past year. It was on these

ccasions that applications were made to the Admiralty, either for new instruments or for




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eveloping the work of the observatory in some other way. After the more official business e inspection was over, the observatory was thrown open to visitors, and hundreds of peo

njoyed on that day the privilege of seeing the national observatory. These annual gatherine happily still continued, and the first Saturday in June is known to be the occasion of onee most interesting reunions of scientific men which takes place in the course of the year.

ry's scientific work was, however, by no means confined to the observatory. He interested

mself largely in expeditions for the observation of eclipses and in projects for theeasurement of arcs on the earth. He devoted much attention to the collection of magneticbservations from various parts of the world. Especially will it be remembered that thercumstances of the transits of Venus, which occurred in 1874 and in 1882, were investigay him, and under his guidance expeditions were sent forth to observe the transits from thocalities in remote parts of the earth where observations most suitable for the determinatiothe sun's distance from the earth could be obtained. The Astronomer Royal also studied

dal phenomena, and he rendered great service to the country in the restoration of theandards of length and weight which had been destroyed in the great fire at the House of

arliament in October, 1834. In the most practical scientific matters his advice was oftenought, and was as cheerfully rendered. Now we find him engaged in an investigation of thegularities of the compass in iron ships, with a view to remedying its defects; now we finm reporting on the best gauge for railways. Among the most generally useful developmenthe observatory must be mentioned the telegraphic method for the distribution of exact

me. By arrangement with the Post Office, the astronomers at Greenwich despatch eachorning a signal from the observatory to London at ten o'clock precisely. By special

pparatus, this signal is thence distributed automatically over the country, so as to enable tme to be known everywhere accurately to a single second. It was part of the same system

at a time ball should be dropped daily at one o'clock at Deal, as well as at other places, foe purpose of enabling ship's chronometers to be regulated.

ry's writings were most voluminous, and no fewer than forty- eight memoirs by him areentioned in the "Catalogue of Scientific Memoirs," published by the Royal Society up to th

ear 1873, and this only included ten years out of an entire life of most extraordinary activiany other subjects besides those of a purely scientific character from time to time engages attention. He wrote, for instance, a very interesting treatise on the Roman invasion of itain, especially with a view of determining the port from which Caesar set forth from Gau

nd the point at which he landed on the British coast. Airy was doubtless led to thisvestigation by his study of the tidal phenomena in the Straits of Dover. Perhaps thestronomer Royal is best known to the general reading public by his excellent lectures ontronomy, delivered at the Ipswich Museum in 1848. This book has passed through many

ditions, and it gives a most admirable account of the manner in which the fundamentaloblems in astronomy have to be attacked.

s years rolled by almost every honour and distinction that could be conferred upon aientific man was awarded to Sir George Airy. He was, indeed, the recipient of other hono




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ot often awarded for scientific distinction. Among these we may mention that in 1875 heceived the freedom of the City of London, "as a recognition of his indefatigable labours intronomy, and of his eminent services in the advancement of practical science, whereby h

as so materially benefited the cause of commerce and civilisation."

ntil his eightieth year Airy continued to discharge his labours at Greenwich with unflaggingnergy. At last, on August 15th, 1881, he resigned the office which he had held so long wit

ch distinction to himself and such benefit to his country. He had married in 1830 theaughter of the Rev. Richard Smith, of Edensor. Lady Airy died in 1875, and three sons andree daughters survived him. One daughter is the wife of Dr. Routh, of Cambridge, and hisher daughters were the constant companions of their father during the declining years ofe. Up to the age of ninety he enjoyed perfect physical health, but an accidental fall whichen occurred was attended with serious results. He died on Saturday, January 2nd, 1892, as buried in the churchyard at Playford.

HAMILTON.

illiam Rowan Hamilton was born at midnight between the 3rd and 4th of August, 1805, atublin, in the house which was then 29, but subsequently 36, Dominick Street. His father,chibald Hamilton, was a solicitor, and William was the fourth of a family of nine. Withference to his descent, it may be sufficient to notice that his ancestors appear to have be

hiefly of gentle Irish families, but that his maternal grandmother was of Scottish birth. Whe was about a year old, his father and mother decided to hand over the education of thehild to his uncle, James Hamilton, a clergyman of Trim, in County Meath. James Hamilton'ster, Sydney, resided with him, and it was in their home that the days of William's childho

ere passed.

Mr. Graves' "Life of Sir William Rowan Hamilton" a series of letters will be found, in whicunt Sydney details the progress of the boy to his mother in Dublin. Probably there is nocord of an infant prodigy more extraordinary than that which these letters contain. At thre

ears old his aunt assured the mother that William is "a hopeful blade," but at that time it ws physical vigour to which she apparently referred; for the proofs of his capacity, which shdduces, related to his prowess in making boys older than himself fly before him. In thecond letter, a month later, we hear that William is brought in to read the Bible for the

urpose of putting to shame other boys double his age who could not read nearly so well.ncle James appears to have taken much pains with William's schooling, but his aunt said tow he picks up everything is astonishing, for he never stops playing and jumping about."hen he was four years and three months old, we hear that he went out to dine at thecar's, and amused the company by reading for them equally well whether the book wasrned upside down or held in any other fashion. His aunt assures the mother that " Willie iost sensible little creature, but at the same time has a great deal of roguery." At four yea

nd five months old he came up to pay his mother a visit in town, and she writes to her sisdescription of the boy;-




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illiam that he could be of little use to him as a tutor, for his pupil was quite as fit to be histor. Eliza Hamilton, by whom this is recorded, adds, "But there is one thing which Boytonould promise to be to him, and that was a FRIEND; and that one proof he would give of tould be that, if ever he saw William beginning to be UPSET by the sensation he would

xcite, and the notice he would attract, he would tell him of it." At the beginning of his collereer he distanced all his competitors in every intellectual pursuit. At his first term

xamination in the University he was first in Classics and first in Mathematics, while he

ceived the Chancellor's prize for a poem on the Ionian Islands, and another for his poem ustace de St. Pierre.

here is abundant testimony that Hamilton had "a heart for friendship formed." Among thearmest of the friends whom he made in these early days was the gifted Maria Edgeworth,ho writes to her sister about "young Mr. Hamilton, an admirable Crichton of eighteen, a reodigy of talents, who Dr. Brinkley says may be a second Newton, quiet, gentle, and simps sister Eliza, to whom he was affectionately attached, writes to him in 1824:--

had been drawing pictures of you in my mind in your study at Cumberland Street withenophon,' &c., on the table, and you, with your most awfully sublime face of thought, nowtting down, and now walking about, at times rubbing your hands with an air of satisfactiond at times bursting forth into some very heroical strain of poetry in an unknown languagend in your own internal solemn ventriloquist-like voice, when you address yourself to theence and solitude of your own room, and indeed, at times, even when your mysterious

oetical addresses are not quite unheard."

his letter is quoted because it refers to a circumstance which all who ever met with Hamilt

ven in his latest years, will remember. He was endowed with two distinct voices, one a higeble, the other a deep bass, and he alternately employed these voices not only in ordinaryonversation, but when he was delivering an address on the profundities of Quaternions to oyal Irish Academy, or on similar occasions. His friends had long grown so familiar with theculiarity that they were sometimes rather surprised to find how ludicrous it appeared torangers.

amilton was fortunate in finding, while still at a very early age, a career open before himhich was worthy of his talents. He had not ceased to be an undergraduate before he was

lled to fill an illustrious chair in his university. The circumstances are briefly as follows.

e have already mentioned that, in 1826, Brinkley was appointed Bishop of Cloyne, and thofessorship of astronomy thereupon became vacant. Such was Hamilton's conspicuous

minence that, notwithstanding he was still an undergraduate, and had only just completeds twenty-first year, he was immediately thought of as a suitable successor to the chair.deed, so remarkable were his talents in almost every direction that had the vacancy beene professorship of classics or of mathematics, of English literature or of metaphysics, of




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odern or of Oriental languages, it seems difficult to suppose that he would not haveccurred to every one as a possible successor. The chief ground, however, on which theends of Hamilton urged his appointment was the earnest of original power which he hadready shown in a research on the theory of Systems of Rays. This profound work created ew branch of optics, and led a few years later to a superb discovery, by which the fame ofuthor became world-wide.

first Hamilton thought it would be presumption for him to apply for so exalted a positionccordingly retired to the country, and resumed his studies for his degree. Other eminentndidates came forward, among them some from Cambridge, and a few of the Fellows froinity College, Dublin, also sent in their claims. It was not until Hamilton received an urgentter from his tutor Boyton, in which he was assured of the favourable disposition of theoard towards his candidature, that he consented to come forward, and on June 16th, 182e was unanimously chosen to succeed the Bishop of Cloyne as Professor of Astronomy in tniversity. The appointment met with almost universal approval. It should, however, be noat Brinkley, whom Hamilton succeeded, did not concur in the general sentiment. No one

ould have formed a higher opinion than he had done of Hamilton's transcendent powers;deed, it was on that very ground that he seemed to view the appointment withsapprobation. He considered that it would have been wiser for Hamilton to have obtainedellowship, in which capacity he would have been able to exercise a greater freedom in hishoice of intellectual pursuits. The bishop seems to have thought, and not without reason,at Hamilton's genius would rather recoil from much of the routine work of an astronomicatablishment. Now that Hamilton's whole life is before us, it is easy to see that the bishopas entirely wrong. It is quite true that Hamilton never became a skilled astronomicalbserver; but the seclusion of the observatory was eminently favourable to those gigantic

bours to which his life was devoted, and which have shed so much lustre, not only onamilton himself, but also on his University and his country.

his early years at Dunsink, Hamilton did make some attempts at a practical use of thelescopes, but he possessed no natural aptitude for such work, while exposure which itvolved seems to have acted injuriously on his health. He, therefore, gradually allowed histention to be devoted to those mathematical researches in which he had already given suomise of distinction. Although it was in pure mathematics that he ultimately won his greame, yet he always maintained and maintained with justice, that he had ample claims to th

le of an astronomer. In his later years he set forth this position himself in a rather strikinganner. De Morgan had written commending to Hamilton's notice Grant's "History of Physicstronomy." After becoming acquainted with the book, Hamilton writes to his friend asllows:--

The book is very valuable, and very creditable to its composer. But your humble servant me pardoned if he finds himself somewhat amused at the title, `History of Physical Astronomom the Earliest Ages to the Middle of the Nineteenth Century,' when he fails to observe anotice of the discoveries of Sir W. R. Hamilton in the theory of the 'Dynamics of the Heaven




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he intimacy between the two correspondents will account for the tone of this letter; and,deed, Hamilton supplies in the lines which follow ample grounds for his complaint. He tellow Jacobi spoke of him in Manchester in 1842 as "le Lagrange de votre pays," and howonkin had said that, "The Analytical Theory of Dynamics as it exists at present is due mainthe labours of La Grange Poisson, Sir W. R. Hamilton, and Jacobi, whose researches on tbject present a series of discoveries hardly paralleled for their elegance and importance i

ny other branch of mathematics." In the same letter Hamilton also alludes to the successhich had attended the applications of his methods in other hands than his own to theucidation of the difficult subject of Planetary Perturbations. Even had his contributions toience amounted to no more than these discoveries, his tenure of the chair would have be

n illustrious one. It happens, however, that in the gigantic mass of his intellectual work thesearches, though intrinsically of such importance, assume what might almost be describe a relative insignificance.

he most famous achievement of Hamilton's earlier years at the observatory was the

scovery of conical refraction. This was one of those rare events in the history of science, ihich a sagacious calculation has predicted a result of an almost startling character,bsequently confirmed by observation. At once this conferred on the young professor a wode renown. Indeed, though he was still only twenty-seven, he had already lived through

mount of intellectual activity which would have been remarkable for a man of threescore an.

multaneously with his growth in fame came the growth of his several friendships. Thereere, in the first place, his scientific friendships with Herschel, Robinson, and many others

th whom he had copious correspondence. In the excellent biography to which I haveferred, Hamilton's correspondence with Coleridge may be read, as can also the letters to dy correspondents, among them being Maria Edgeworth, Lady Dunraven, and Ladyampbell. Many of these sheets relate to literary matters, but they are largely intermingledith genial pleasantry, and serve at all events to show the affection and esteem with whichas regarded by all who had the privilege of knowing him. There are also the letters to thesters whom he adored, letters brimming over with such exalted sentiment, that mostdinary sisters would be tempted to receive them with a smile in the excessively improbab

vent of their still more ordinary brothers attempting to pen such effusions. There are also

dications of letters to and from other young ladies who from time to time were the objectamilton's tender admiration. We use the plural advisedly, for, as Mr. Graves has set forth,amilton's love affairs pursued a rather troubled course. The attention which he lavished onne or two fair ones was not reciprocated, and even the intense charms of mathematicalscovery could not assuage the pangs which the disappointed lover experienced. At last heached the haven of matrimony in 1833, when he was married to Miss Bayly. Of his marriee Hamilton said, many years later to De Morgan, that it was as happy as he expected, andappier than he deserved. He had two sons, William and Archibald, and one daughter, Heleho became the wife of Archdeacon O'Regan.




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he most remarkable of Hamilton's friendships in his early years was unquestionably that wordsworth. It commenced with Hamilton's visit to Keswick; and on the first evening, whene poet met the young mathematician, an incident occurred which showed the mutualterest that was aroused. Hamilton thus describes it in a letter to his sister Eliza:--

He (Wordsworth) walked back with our party as far as their lodge, and then, on our biddin

rs. Harrison good-night, I offered to walk back with him while my party proceeded to theotel. This offer he accepted, and our conversation had become so interesting that when wad arrived at his home, a distance of about a mile, he proposed to walk back with me on may to Ambleside, a proposal which you may be sure I did not reject; so far from it that whe came to turn once more towards his home I also turned once more along with him. It wery late when I reached the hotel after all this walking."

amilton also submitted to Wordsworth an original poem, entitled "It Haunts me Yet." Theply of Wordsworth is worth repeating:--

With a safe conscience I can assure you that, in my judgment, your verses are animated we poetic spirit, as they are evidently the product of strong feeling. The sixth and seventhanzas affected me much, even to the dimming of my eyes and faltering of my voice whileas reading them aloud. Having said this, I have said enough. Now for the per contra. Youll not, I am sure, be hurt when I tell you that the workmanship (what else could be

xpected from so young a writer?) is not what it ought to be. . .

My household desire to be remembered to you in no formal way. Seldom have I parted--

ever, I was going to say--with one whom after so short an acquaintance I lost sight of witore regret. I trust we shall meet again."

he further affectionate intercourse between Hamilton and Wordsworth is fully set forth, anHamilton's latest years a recollection of his "Rydal hours" was carefully treasured and

equently referred to. Wordsworth visited Hamilton at the observatory, where a beautifulady path in the garden is to the present day spoken of as "Wordsworth's Walk."

was the practice of Hamilton to produce a sonnet on almost every occasion which admittpoetical treatment, and it was his delight to communicate his verses to his friends all rouhen Whewell was producing his "Bridgewater Treatises," he writes to Hamilton in 1833:--

Your sonnet which you showed me expressed much better than I could express it the feelith which I tried to write this book, and I once intended to ask your permission to prefix t

onnet to my book, but my friends persuaded me that I ought to tell my story in my ownose, however much better your verse might be."




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he first epoch-marking contribution to Theoretical Dynamics after the time of Newton wasndoubtedly made by Lagrange, in his discovery of the general equations of Motion. The neeat step in the same direction was that taken by Hamilton in his discovery of a still more

omprehensive method. Of this contribution Hamilton writes to Whewell, March 31st, 1834:

As to my late paper, a day or two ago sent off to London, it is merely mathematical andeductive. I ventured, indeed, to call it the 'Mecanique Analytique' of Lagrange, 'a scientific

oem'; and spoke of Dynamics, or the Science of Force, as treating of 'Power acting by Lawpace and Time.' In other respects it is as unpoetical and unmetaphysical as my gravestends could desire."

may well be doubted whether there is a more beautiful chapter in the whole of athematical philosophy than that which contains Hamilton's dynamical theory. It is disfiguy no tedious complexity of symbols; it condescends not to any particular problems; it is anmbracing theory, which gives an intellectual grasp of the most appropriate method forscovering the result of the application of force to matter. It is the very generality of this

octrine which has somewhat impeded the applications of which it is susceptible. Thexigencies of examinations are partly responsible for the fact that the method has not becoore familiar to students of the higher mathematics. An eminent professor has complainedat Hamilton's essay on dynamics was of such an extremely abstract character, that he foumself unable to extract from it problems suitable for his examination papers.

he following extract is from a letter of Professor Sylvester to Hamilton, dated 20th of eptember, 1841. It will show how his works were appreciated by so consummate aathematician as the writer:--

Believe me, sir, it is not the least of my regrets in quitting this empire to feel that I forego sual occasion of meeting those masters of my art, yourself chief amongst the number,hose acquaintance, whose conversation, or even notice, have in themselves the power tospire, and almost to impart fresh vigour to the understanding, and the courage and faiththout which the efforts of invention are in vain. The golden moments I enjoyed under yo

ospitable roof at Dunsink, or moments such as they were, may probably never again fall toy lot.

a vast distance, and in an humble eminence, I still promise myself the calm satisfaction observing your blazing course in the elevated regions of discovery. Such national honour asou are able to confer on your country is, perhaps, the only species of that luxury for the rimean what is termed one's glory) which is not bought at the expense of the comforts of illion."

he study of metaphysics was always a favourite recreation when Hamilton sought for ahange from the pursuit of mathematics. In the year 1834 we find him a diligent student of




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ant; and, to show the views of the author of Quaternions and of Algebra as the Science ofure Time on the "Critique of the Pure Reason," we quote the following letter, dated 18th oly, 1834, from Hamilton to Viscount Adare:--

have read a large part of the 'Critique of the Pure Reason,' and find it wonderfully clear, enerally quite convincing. Notwithstanding some previous preparation from Berkeley, andom my own thoughts, I seem to have learned much from Kant's own statement of his view

'Space and Time.' Yet, on the whole, a large part of my pleasure consists in recognisingrough Kant's works, opinions, or rather views, which have been long familiar to myself,though far more clearly and systematically expressed and combined by him.. . . Kant is, Iink, much more indebted than he owns, or, perhaps knows, to Berkeley, whom he calls beer, `GUTEM Berkeley'. . . as it were, `good soul, well meaning man,' who was able for aat to shake to its centre the world of human thought, and to effect a revolution among th

arly consequences of which was the growth of Kant himself."

several meetings of the British Association Hamilton was a very conspicuous figure.

specially was this the case in 1835, when the Association met in Dublin, and when Hamiltoough then but thirty years old, had attained such celebrity that even among a very brillian

athering his name was perhaps the most renowned. A banquet was given at Trinity Colleghonour of the meeting. The distinguished visitors assembled in the Library of the Univers

he Earl of Mulgrave, then Lord Lieutenant of Ireland, made this the opportunity of conferrn Hamilton the honour of knighthood, gracefully adding, as he did so: "I but set the royal,nd therefore the national mark, on a distinction already acquired by your genius andbours."

he banquet followed, writes Mr. Graves. "It was no little addition to the honour Hamilton hready received that, when Professor Whewell returned thanks for the toast of the UniversCambridge, he thought it appropriate to add the words, 'There was one point which

rongly pressed upon him at that moment: it was now one hundred and thirty years since eat man in another Trinity College knelt down before his sovereign, and rose up Sir Isaacewton.' The compliment was welcomed by immense applause."

more substantial recognition of the labours of Hamilton took place subsequently. He thusescribes it in a letter to Mr. Graves of 14th of November, 1843:--

The Queen has been pleased--and you will not doubt that it was entirely unsolicited, andven unexpected, on my part--'to express her entire approbation of the grant of a pension wo hundred pounds per annum from the Civil List' to me for scientific services. The lettersom Sir Robert Peel and from the Lord Lieutenant of Ireland in which this grant has beenommunicated or referred to have been really more gratifying to my feelings than the addit

my income, however useful, and almost necessary, that may have been."




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he circumstances we have mentioned might lead to the supposition that Hamilton was thethe zenith of his fame but this was not so. It might more truly be said, that his

chievements up to this point were rather the preliminary exercises which fitted him for thegantic task of his life. The name of Hamilton is now chiefly associated with his memorablevention of the calculus of Quaternions. It was to the creation of this branch of mathematiat the maturer powers of his life were devoted; in fact he gives us himself an illustration

ow completely habituated he became to the new modes of thought which Quaternions

iginated. In one of his later years he happened to take up a copy of his famous paper onynamics, a paper which at the time created such a sensation among mathematicians, andhich is at this moment regarded as one of the classics of dynamical literature. He read, hells us, his paper with considerable interest, and expressed his feelings of gratification thatund himself still able to follow its reasoning without undue effort. But it seemed to him ale time as a work belonging to an age of analysis now entirely superseded.

order to realise the magnitude of the revolution which Hamilton has wrought in thepplication of symbols to mathematical investigation, it is necessary to think of what Hamilt

d beside the mighty advance made by Descartes. To describe the character of theuaternion calculus would be unsuited to the pages of this work, but we may quote anteresting letter, written by Hamilton from his deathbed, twenty-two years later, to his sonchibald, in which he has recorded the circumstances of the discovery:--

deed, I happen to be able to put the finger of memory upon the year and month--Octobe843--when having recently returned from visits to Cork and Parsonstown, connected with eeting of the British Association, the desire to discover the laws of multiplication referred gained with me a certain strength and earnestness which had for years been dormant, bu

as then on the point of being gratified, and was occasionally talked of with you. Everyorning in the early part of the above- cited month, on my coming down to breakfast, youhen) little brother William Edwin, and yourself, used to ask me, 'Well papa, can you multipplets?' Whereto I was always obliged to reply, with a sad shake of the head: 'No, I can on

DD and subtract them,'

ut on the 16th day of the same month--which happened to be Monday, and a Council daye Royal Irish Academy--I was walking in to attend and preside, and your mother wasalking with me along the Royal Canal, to which she had perhaps driven; and although she

lked with me now and then, yet an UNDERCURRENT of thought was going on in my mindhich gave at last a RESULT, whereof it is not too much to say that I felt AT ONCE the

mportance. An ELECTRIC circuit seemed to CLOSE; and a spark flashed forth the herald (aORESAW IMMEDIATELY) of many long years to come of definitely directed thought and wy MYSELF, if spared, and, at all events, on the part of OTHERS if I should even be allowedve long enough distinctly to communicate the discovery. Nor could I resist the impulse--nphilosophical as it may have been--to cut with a knife on a stone of Brougham Bridge as assed it, the fundamental formula which contains the SOLUTION of the PROBLEM, but, of ourse, the inscription has long since mouldered away. A more durable notice remains,




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owever, on the Council Books of the Academy for that day (October 16, 1843), which recoe fact that I then asked for and obtained leave to read a Paper on 'Quaternions,' at the Fieneral Meeting of the Session; which reading took place accordingly, on Monday, the 13thovember following."

riting to Professor Tait, Hamilton gives further particulars of the same event. And again intter to the Rev. J. W. Stubbs:--

To-morrow will be the fifteenth birthday of the Quaternions. They started into life full-grown the 16th October, 1843, as I was walking with Lady Hamilton to Dublin, and came up toougham Bridge--which my boys have since called Quaternion Bridge. I pulled out a

ocketbook which still exists, and made entry, on which at the very moment I felt that it me worth my while to expend the labour of at least ten or fifteen years to come. But then itir to say that this was because I felt a problem to have been at that moment solved, antellectual want relieved which had haunted me for at least fifteen years before.

ut did the thought of establishing such a system, in which geometrically opposite facts--amely, two lines (or areas) which are opposite IN SPACE give ALWAYS a positive product-ver come into anybody's head till I was led to it in October, 1843, by trying to extend my oeory of algebraic couples, and of algebra as the science of pure time? As to my regarding

eometrical addition of lines as equivalent to composition of motions (and as performed by me rules), that is indeed essential in my theory but not peculiar to it; on the contrary, I a

nly one of many who have been led to this view of addition."

grims in future ages will doubtless visit the spot commemorated by the invention of

uaternions. Perhaps as they look at that by no means graceful structure Quaternion Bridgey will regret that the hand of some Old Mortality had not been occasionally employed in

utting the memorable inscription afresh. It is now irrecoverably lost.

was ten years after the discovery that the great volume appeared under the title of ectures on Quaternions," Dublin, 1853. The reception of this work by the scientific world ch as might have been expected from the extraordinary reputation of its author, and the

ovelty and importance of the new calculus. His valued friend, Sir John Herschel, writes to that style of which he was a master:--

Now, most heartily let me congratulate you on getting out your book--on having foundterance, ore rotundo, for all that labouring and seething mass of thought which has beenom time to time sending out sparks, and gleams, and smokes, and shaking the soil aboutou; but now breaks into a good honest eruption, with a lava stream and a shower of rtilizing ashes.

etaphor and simile apart, there is work for a twelve-month to any man to read such a boo




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nd for half a lifetime to digest it, and I am glad to see it brought to a conclusion."

e may also record Hamilton's own opinion expressed to Humphrey Lloyd:--

n general, although in one sense I hope that I am actually growing modest about theuaternions, from my seeing so many peeps and vistas into future expansions of theirinciples, I still must assert that this discovery appears to me to be as important for the

iddle of the nineteenth century as the discovery of fluxions was for the close of theventeenth."

artholomew Lloyd died in 1837. He had been the Provost of Trinity College, and the Presidthe Royal Irish Academy. Three candidates were put forward by their respective friends fe vacant Presidency. One was Humphrey Lloyd, the son of the late Provost, and the twohers were Hamilton and Archbishop Whately. Lloyd from the first urged strongly the claimHamilton, and deprecated the putting forward of his own name. Hamilton in like manner

esired to withdraw in favour of Lloyd. The wish was strongly felt by many of the Fellows o

e College that Lloyd should be elected, in consequence of his having a more intimatesociation with collegiate life than Hamilton; while his scientific eminence was world-wide.

he election ultimately gave Hamilton a considerable majority over Lloyd, behind whom thechbishop followed at a considerable distance. All concluded happily, for both Lloyd and thchbishop expressed, and no doubt felt, the pre-eminent claims of Hamilton, and both of em cordially accepted the office of a Vice-President, to which, according to the constitutiothe Academy, it is the privilege of the incoming President to nominate.

another chapter I have mentioned as a memorable episode in astronomical history, that

Herschel went for a prolonged sojourn to the Cape of Good Hope, for the purpose of bmitting the southern skies to the same scrutiny with the great telescope that his father ven to the northern skies. The occasion of Herschel's return after the brilliant success of hnterprise, was celebrated by a banquet. On June 15th, 1838, Hamilton was assigned the honour of proposing the health of Herschel. This banquet is otherwise memorable inamilton's career as being one of the two occasions in which he was in the company of histimate friend De Morgan.

the year 1838 a scheme was adopted by the Royal Irish Academy for the award of meda

the authors of papers which appeared to possess exceptionally high merit. At the institutthe medal two papers were named in competition for the prize. One was Hamilton's

Memoir on Algebra, as the Science of Pure Time." The other was Macullagh's paper on theaws of Crystalline Reflection and Refraction." Hamilton expresses his gratification that,ainly in consequence of his own exertions, he succeeded in having the medal awarded toacullagh rather than to himself. Indeed, it would almost appear as if Hamilton had procurletter from Sir J. Herschel, which indicated the importance of Macullagh's memoir in such ay as to decide the issue. It then became Hamilton's duty to award the medal from the ch




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nd to deliver an address in which he expressed his own sense of the excellence of acullagh's scientific work. It is the more necessary to allude to these points, because in thhole of his scientific career it would seem that Macullagh was the only man with whomamilton had ever even an approach to a dispute about priority. The incident referred to toace in connection with the discovery of conical refraction, the fame of which Macullagh mpreposterous attempt to wrest from Hamilton. This is evidently alluded to in Hamilton's lethe Marquis of Northampton, dated June 28th, 1838, in which we read:--

nd though some former circumstances prevented me from applying to the person thusstinguished the sacred name of FRIEND, I had the pleasure of doing justice...to his hightellectual merits...I believe he was not only gratified but touched, and may, perhaps, regae in future with feelings more like those which I long to entertain towards him."

amilton was in the habit, from time to time, of commencing the keeping of a journal, but oes not appear to have been systematically conducted. Whatever difficulties the biographeay have experienced from its imperfections and irregularities, seem to be amply

ompensated for by the practice which Hamilton had of preserving copies of his letters, andven of comparatively insignificant memoranda. In fact, the minuteness with which apparenvial matters were often noted down appears almost whimsical. He frequently made aemorandum of the name of the person who carried a letter to the post, and of the hour inhich it was despatched. On the other hand, the letters which he received were also carefueserved in a mighty mass of manuscripts, with which his study was encumbered, and withich many other parts of the house were not unfrequently invaded. If a letter was laid asir a few hours, it would become lost to view amid the seething mass of papers, though

ccasionally, to use his own expression, it might be seen "eddying" to the surface in some

ter disturbance.

he great volume of "Lectures on Quaternions" had been issued, and the author had receive honours which the completion of such a task would rightfully bring him. The publication

n immortal work does not, however, necessarily provide the means for paying the printer'sl. The printing of so robust a volume was necessarily costly; and even if all the copies cou

e sold, which at the time did not seem very likely, they would hardly have met the inevitabxpenses. The provision of the necessary funds was, therefore, a matter for consideration.he Board of Trinity College had already contributed 200 pounds to the printing, but yet

nother hundred was required. Even the discoverer of Quaternions found this a source of uch anxiety. However, the board, urged by the representation of Humphrey Lloyd, now oits members, and, as we have already seen, one of Hamilton's staunchest friends, relieve

m of all liability. We may here note that, notwithstanding the pension which Hamiltonnjoyed in addition to the salary of his chair, he seems always to have been in some whatraitened circumstances, or, to use his own words in one of his letters to De Morgan, "Thoot an embarrassed man, I am anything rather than a rich one." It appears that,otwithstanding the world-wide fame of Hamilton's discoveries, the only profit in a pecuniarnse that he ever obtained from any of his works was by the sale of what he called his




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osian Game. Some enterprising publisher, on the urgent representations of one of Hamiltoends in London, bought the copyright of the Icosian Game for 25 pounds. Even this little

peculation proved unfortunate for the purchaser, as the public could not be induced to take necessary interest in the matter.

ter the completion of his great book, Hamilton appeared for awhile to permit himself aeater indulgence than usual in literary relaxations. He had copious correspondence with h

timate friend, Aubrey de Vere, and there were multitudes of letters from those troops of ends whom it was Hamilton's privilege to possess. He had been greatly affected by the

eath of his beloved sister Eliza, a poetess of much taste and feeling. She left to him her mapers to preserve or to destroy, but he said it was only after the expiration of four years oourning that he took courage to open her pet box of letters.

he religious side of Hamilton's character is frequently illustrated in these letters; especiallyis brought out in the correspondence with De Vere, who had seceded to the Church of ome. Hamilton writes, August 4, 1855:--

f, then, it be painfully evident to both, that under such circumstances there CANNOTwhatever we may both DESIRE) be NOW in the nature of things, or of minds, the sameegree of INTIMACY between us as of old; since we could no longer TALK with the sameegree of unreserve on every subject which happened to present itself, but MUST, from themplest instincts of courtesy, be each on his guard not to say what might be offensive, or, ast, painful to the other; yet WE were ONCE so intimate, an retain still, and, as I trust, shways retain, so much of regard and esteem and appreciation for each other, made tendero many associations of my early youth and your boyhood, which can never be forgotten by

ther of us, that (as times go) TWO OR THREE VERY RESPECTABLE FRIENDSHIPS mightasily be carved out from the fragments of our former and ever-to-be-remembered INTIMAwould be no exaggeration to quote the words: 'Heu! quanto minus est cum reliquis versa

uam tui meminisse!'"

1858 a correspondence on the subject of Quaternions; commenced between Professor Tnd Sir William Hamilton. It was particularly gratifying to the discoverer that so competent aathematician as Professor Tait should have made himself acquainted with the new calculuis, of course, well known that Professor Tait subsequently brought out a most valuable

ementary treatise on Quaternions, to which those who are anxious to become acquaintedth the subject will often turn in preference to the tremendous work of Hamilton.

the year 1861 gratifying information came to hand of the progress which the study of uaternions was making abroad. Especially did the subject attract the attention of thatccomplished mathematician, Moebius, who had already in his "Barycentrische Calculus" bed to conceptions which bore more affinity to Quaternions than could be found in the writiany other mathematician. Such notices of his work were always pleasing to Hamilton, an




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ey served, perhaps, as incentives to that still closer and more engrossing labour by whichecame more and more absorbed. During the last few years of his life he was observed to ven more of a recluse than he had hitherto been. His powers of long and continuous studyemed to grow with advancing years, and his intervals of relaxation, such as they were,

ecame more brief and more infrequent.

was not unusual for him to work for twelve hours at a stretch. The dawn would frequent

rprise him as he looked up to snuff his candles after a night of fascinating labour at originsearch. Regularity in habits was impossible to a student who had prolonged fits of what hlled his mathematical trances. Hours for rest and hours for meals could only be snatched e occasional the lucid intervals between one attack of Quaternions and the next. When

ungry, he would go to see whether any thing could be found on the sideboard; when thirse would visit the locker, and the one blemish in the man's personal character is that thesetter visits were sometimes paid too often.

s an example of one of Hamilton's rare diversions from the all- absorbing pursuit of

uaternions, we find that he was seized with curiosity to calculate back to the date of theegira, which he found on the 15th July, 622. He speaks of the satisfaction with which hecertained subsequently that Herschel had assigned precisely the same date. Metaphysicsmained also, as it had ever been, a favourite subject of Hamilton's readings and meditatio

nd of correspondence with his friends. He wrote a very long letter to Dr. Ingleby on thebject of his "Introduction to Metaphysics." In it Hamilton alludes, as he has done also inher places, to a peculiarity of his own vision. It was habitual to him, by some defect in th

orrelation of his eyes, to see always a distinct image with each; in fact, he speaks of themarkable effect which the use of a good stereoscope had on his sensations of vision. It w

en, for the first time, that he realised how the two images which he had always seentherto would, under normal circumstances, be blended into one. He cites this fact as bearn the phenomena of binocular vision, and he draws from it the inference that the necessitnocular vision for the correct appreciation of distance is unfounded. "I am quite sure," heys, "that I SEE DISTANCE with EACH EYE SEPARATELY."

he commencement of 1865, the last year of his life saw Hamilton as diligent as ever, andorresponding with Salmon and Cayley. On April 26th he writes to a friend to say, that hisealth has not been good for years past, and that so much work has injured his constitution

nd he adds, that it is not conducive to good spirits to find that he is accumulating anothereavy bill with the printer for the publication of the "Elements." This was, indeed, up to theay of his death, a cause for serious anxiety. It may, however, be mentioned that the wholost, which amounted to nearly 500 pounds, was, like that of the previous volume, ultimateorne by the College. Contrary to anticipation, the enterprise, even in a pecuniary sense,nnot have been a very unprofitable one. The whole edition has long been out of print, an much as 5 pounds has since been paid for a single copy.

was on the 9th of May, 1865, that Hamilton was in Dublin for the last time. A few days la




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e had a violent attack of gout, and on the 4th of June he became alarmingly ill, and on theext day had an attack of epileptic convulsions. However, he slightly rallied, so that before nd of the month he was again at work at the "Elements." A gratifying incident brightenedome of the last days of his life. The National Academy of Science in America had then beest formed. A list of foreign Associates had to be chosen from the whole world, and ascussion took place as to what name should be placed first on the list. Hamilton wasformed by private communication that this great distinction was awarded to him by a

ajority of two-thirds.

August he was still at work on the table of contents of the "Elements," and one of his vetest efforts was his letter to Mr. Gould, in America, communicating his acknowledgementse honour which had been just conferred upon him by the National Academy. On the 2nd

eptember Mr. Graves went to the observatory, in response to a summons, and the greatathematician at once admitted to his friend that he felt the end was approaching. Heentioned that he had found in the 145th Psalm a wonderfully suitable expression of hisoughts and feelings, and he wished to testify his faith and thankfulness as a Christian by

artaking of the Lord's Supper. He died at half-past two on the afternoon of the 2nd of eptember, 1865, aged sixty years and one month. He was buried in Mount Jerome Cemeten the 7th of September.

any were the letters and other more public manifestations of the feelings awakened byamilton's death. Sir John Herschel wrote to the widow:--

ermit me only to add that among the many scientific friends whom time has deprived meere has been none whom I more deeply lament, not only for his splendid talents, but for

xcellence of his disposition and the perfect simplicity of his manners--so great, and yetevoid of pretensions."

e Morgan, his old mathematical crony, as Hamilton affectionately styled him, also wrote toady Hamilton:--

have called him one of my dearest friends, and most truly; for I know not how much longan twenty-five years we have been in intimate correspondence, of most friendly agreemedisagreement, of most cordial interest in each other. And yet we did not know each othe

ces. I met him about 1830 at Babbage's breakfast table, and there for the only time in ouves we conversed. I saw him, a long way off, at the dinner given to Herschel (about 1838)s return from the Cape and there we were not near enough, nor on that crowded day coue get near enough, to exchange a word. And this is all I ever saw, and, so it has pleasedod, all I shall see in this world of a man whose friendly communications were among myeatest social enjoyments, and greatest intellectual treats."

here is a very interesting memoir of Hamilton written by De Morgan, in the "Gentleman's




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agazine" for 1866, in which he produces an excellent sketch of his friend, illustrated byersonal reminiscences and anecdotes. He alludes, among other things, to the picturesqueonfusion of the papers in his study. There was some sort of order in the mass, discernibleowever, by Hamilton alone, and any invasion of the domestics, with a view to tidying up,ould throw the mathematician as we are informed, into "a good honest thundering passio

ardly any two men, who were both powerful mathematicians, could have been more

ssimilar in every other respect than were Hamilton and De Morgan. The highly poeticalmperament of Hamilton was remarkably contrasted with the practical realism of De Morgaamilton sends sonnets to his friend, who replies by giving the poet advice about making hll. The metaphysical subtleties, with which Hamilton often filled his sheets, did not seem t

ave the same attraction for De Morgan that he found in battles about the quantification ofe Predicate. De Morgan was exquisitely witty, and though his jokes were always apprecia

y his correspondent, yet Hamilton seldom ventured on anything of the same kind in reply;deed his rare attempts at humour only produced results of the most ponderous descriptiout never were two scientific correspondents more perfectly in sympathy with each other.

amilton's work on Quaternions, his labours in Dynamics, his literary tastes, his metaphysicnd his poetry, were all heartily welcomed by his friend, whose letters in reply invariablyvince the kindliest interest in all Hamilton's concerns. In a similar way De Morgan's letters amilton always met with a heartfelt response.

ike for the memory of Hamilton, for the credit of his University, and for the benefit of ience, let us hope that a collected edition of his works will ere long appear--a collectionhich shall show those early achievements in splendid optical theory, those achievements os more mature powers which made him the Lagrange of his country, and finally those

eations of the Quaternion Calculus by which new capabilities have been bestowed on theuman intellect.

LE VERRIER.

he name of Le Verrier is one that goes down to fame on account of very different discoveom those which have given renown to several of the other astronomers whom we haveentioned. We are sometimes apt to identify the idea of an astronomer with that of a manho looks through a telescope at the stars; but the word astronomer has really much wider

gnificance. No man who ever lived has been more entitled to be designated an astronomean Le Verrier, and yet it is certain that he never made a telescopic discovery of any kind.deed, so far as his scientific achievements have been concerned, he might never haveoked through a telescope at all.

or the full interpretation of the movements of the heavenly bodies, mathematical knowledthe most advanced character is demanded. The mathematician at the outset calls upon ttronomer who uses the instruments in the observatory, to ascertain for him at various tim




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e exact positions occupied by the sun, the moon, and the planets. These observations,btained with the greatest care, and purified as far as possible from the errors by which theay be affected form, as it were, the raw material on which the mathematician exercises h

kill. It is for him to elicit from the observed places the true laws which govern the movemethe heavenly bodies. Here is indeed a task in which the highest powers of the human

tellect may be worthily employed.

mong those who have laboured with the greatest success in the interpretation of thebservations made with instruments of precision, Le Verrier holds a highly honoured place. m it has been given to provide a superb illustration of the success with which the mind ofan can penetrate the deep things of Nature.

he illustrious Frenchman, Urban Jean Joseph Le Verrier, was born on the 11th March, 181St. Lo, in the department of Manche. He received his education in that famous school for

ducation in the higher branches of science, the Ecole Polytechnique, and acquired thereonsiderable fame as a mathematician. On leaving the school Le Verrier at first purposed to

evote himself to the public service, in the department of civil engineering; and it is worthyote that his earliest scientific work was not in those mathematical researches in which he wtimately to become so famous. His duties in the engineering department involved practica

hemical research in the laboratory. In this he seems to have become very expert, andobably fame as a chemist would have been thus attained, had not destiny led him into

nother direction. As it was, he did engage in some original chemical research. His firstontributions to science were the fruits of his laboratory work; one of his papers was on theombination of phosphorus and hydrogen, and another on the combination of phosphorus axygen.

s mathematical labours at the Ecole Polytechnique had, however, revealed to Le Verrier te was endowed with the powers requisite for dealing with the subtlest instruments of athematical analysis. When he was twenty-eight years old, his first great astronomicalvestigation was brought forth. It will be necessary to enter into some explanation as to thature of this, inasmuch as it was the commencement of the life- work which he was toursue.

but a single planet revolved around the sun, then the orbit of that planet would be an

ipse, and the shape and size, as well as the position of the ellipse, would never alter. Onevolution after another would be traced out, exactly in the same manner, in compliance wie force continuously exerted by the sun. Suppose, however, that a second planet betroduced into the system. The sun will exert its attraction on this second planet also, and ll likewise describe an orbit round the central globe. We can, however, no longer assert te orbit in which either of the planets moves remains exactly an ellipse. We may, indeed,sume that the mass of the sun is enormously greater than that of either of the planets. Inis case the attraction of the sun is a force of such preponderating magnitude, that the act

ath of each planet remains nearly the same as if the other planet were absent. But it is




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mpossible for the orbit of each planet not to be affected in some degree by the attraction oe other planet. The general law of nature asserts that every body in space attracts everyher body. So long as there is only a single planet, it is the single attraction between the s

nd that planet which is the sole controlling principle of the movement, and in consequencethe ellipse is described. But when a second planet is introduced, each of the two bodies isot only subject to the attraction of the sun, but each one of the planets attracts the other.true that this mutual attraction is but small, but, nevertheless, it produces some effect. It

isturbs," as the astronomer says, the elliptic orbit which would otherwise have beenursued. Hence it follows that in the actual planetary system where there are several planesturbing each other, it is not true to say that the orbits are absolutely elliptic.

the same time in any single revolution a planet may for most practical purposes be said te actually moving in an ellipse. As, however, time goes on, the ellipse gradually varies. Itters its shape, it alters its plane, and it alters its position in that plane. If, therefore, we wstudy the movements of the planets, when great intervals of time are concerned, it is

ecessary to have the means of learning the nature of the movement of the orbit in

onsequence of the disturbances it has experienced.

e may illustrate the matter by supposing the planet to be running like a railway engine onack which has been laid in a long elliptic path. We may suppose that while the planet isoursing along, the shape of the track is gradually altering. But this alteration may be so sloat it does not appreciably affect the movement of the engine in a single revolution. We caso suppose that the plane in which the rails have been laid has a slow oscillation in level,nd that the whole orbit is with more or less uniformity moved slowly about in the plane.

short periods of time the changes in the shapes and positions of the planetary orbits, inonsequence of their mutual attractions, are of no great consequence. When, however, weing thousands of years into consideration, then the displacements of the planetary orbitstain considerable dimensions, and have, in fact, produced a profound effect on the system

is of the utmost interest to investigate the extent to which one planet can affect another rtue of their mutual attractions. Such investigations demand the exercise of the highestathematical gifts. But not alone is intellectual ability necessary for success in such inquiriemust be united with a patient capacity for calculations of an arduous type, protracted, as

ey frequently have to be, through many years of labour. Le Verrier soon found in theseofound inquiries adequate scope for the exercise of his peculiar gifts. His first importanttronomical publication contained an investigation of the changes which the orbits of sevethe planets, including the earth, have undergone in times past, and which they will undetimes to come.

s an illustration of these researches, we may take the case of the planet in which we are, ourse, especially interested, namely, the earth, and we can investigate the changes which,




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e lapse of time, the earth's orbit has undergone, in consequence of the disturbance to whhas been subjected by the other planets. In a century, or even in a thousand years, there

ut little recognisable difference in the shape of the track pursued by the earth. Vast periodtime are required for the development of the large consequences of planetary perturbati

e Verrier has, however, given us the particulars of what the earth's journey through spaceas been at intervals of 20,000 years back from the present date. His furthest calculationrows our glance back to the state of the earth's track 100,000 years ago, while, with a

ound forward, he shows us what the earth's orbit is to be in the future, at successivetervals of 20,000 years, till a date is reached which is 100,000 years in advance Of A.D.800.

he talent which these researches displayed brought Le Verrier into notice. At that time thearis Observatory was presided over by Arago, a SAVANT who occupies a distinguishedosition in French scientific annals. Arago at once perceived that Le Verrier was just the maho possessed the qualifications suitable for undertaking a problem of great importance anfficulty that had begun to force itself on the attention of astronomers. What this great

oblem was, and how astonishing was the solution it received, must now be considered.

ver since Herschel brought himself into fame by his superb discovery of the great planetranus, the movements of this new addition to the solar system were scrutinized with carend attention. The position of Uranus was thus accurately determined from time to time. Atngth, when sufficient observations of this remote planet had been brought together, theute which the newly-discovered body pursued through the heavens was ascertained byose calculations with which astronomers are familiar. It happens, however, that Uranus

ossesses a superficial resemblance to a star. Indeed the resemblance is so often deceptive

at long ere its detection as a planet by Herschel, it had been observed time after time bykilful astronomers, who little thought that the star-like point at which they looked wasnything but a star. From these early observations it was possible to determine the track ofranus, and it was found that the great planet takes a period of no less than eighty-four yeaccomplish a circuit. Calculations were made of the shape of the orbit in which it revolve

efore its discovery by Herschel, and these were compared with the orbit which observationowed the same body to pursue in those later years when its planetary character was knocould not, of course, be expected that the orbit should remain unaltered; the fact that theat planets Jupiter and Saturn revolve in the vicinity of Uranus must necessarily imply tha

e orbit of the latter undergoes considerable changes. When, however, due allowance haseen made for whatever influence the attraction of Jupiter and Saturn, and we may add of arth and all the other Planets, could possibly produce, the movements of Uranus were stillexplicable. It was perfectly obvious that there must be some other influence at work besidat which could be attributed to the planets already known.

stronomers could only recognise one solution of such a difficulty. It was impossible to douat there must be some other planet in addition to the bodies at that time known, and thae perturbations of Uranus hitherto unaccounted for, were due to the disturbances caused




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e action of this unknown planet. Arago urged Le Verrier to undertake the great problem oarching for this body, whose theoretical existence seemed demonstrated. But the conditiothe search were such that it must needs be conducted on principles wholly different from

ny search which had ever before been undertaken for a celestial object. For this was not ase in which mere survey with a telescope might be expected to lead to the discovery.

ertain facts might be immediately presumed with reference to the unknown object. There

ould be no doubt that the unknown disturber of Uranus must be a large body with a massxceeding that of the earth. It was certain, however, that it must be so distant that it couldnly appear from our point of view as a very small object. Uranus itself lay beyond the rang

almost beyond the range, of unassisted vision. It could be shown that the planet by whice disturbance was produced revolved in an orbit which must lie outside that of Uranus. Itemed thus certain that the planet could not be a body visible to the unaided eye. Indeed

ad it been at all conspicuous its planetary character would doubtless have been detectedges ago. The unknown body must therefore be a planet which would have to be sought foy telescopic aid.

here is, of course, a profound physical difference between a planet and a star, for the starluminous sun, and the planet is merely a dark body, rendered visible by the sunlight whiclls upon it. Notwithstanding that a star is a sun thousands of times larger than the planet

nd millions of times more remote, yet it is a singular fact that telescopic planets possess ausory resemblance to the stars among which their course happens to lie. So far as actualppearance goes, there is indeed only one criterion by which a planet of this kind can bescriminated from a star. If the planet be large enough the telescope will show that itossesses a disc, and has a visible and measurable circular outline. This feature a star does

ot exhibit. The stars are indeed so remote that no matter how large they may be intrinsicaey only exhibit radiant points of light, which the utmost powers of the telescope fail toagnify into objects with an appreciable diameter. The older and well-known planets, suchpiter and Mars, possess discs, which, though not visible to the unaided eye, were clearly

nough discernible with the slightest telescopic power. But a very remote planet like Uranuough it possessed a disc large enough to be quickly appreciated by the consummate

bserving skill of Herschel, was nevertheless so stellar in its appearance, that it had beenbserved no fewer than seventeen times by experienced astronomers prior to Herschel. Inach case the planetary nature of the object had been overlooked, and it had been taken fo

anted that it was a star. It presented no difference which was sufficient to arrest attentio

s the unknown body by which Uranus was disturbed was certainly much more remote tharanus, it seemed to be certain that though it might show a disc perceptible to very closespection, yet that the disc must be so minute as not to be detected except with extremere. In other words, it seemed probable that the body which was to be sought for could noadily be discriminated from a small star, to which class of object it bore a superficialsemblance, though, as a matter of fact, there was the profoundest difference between th

wo bodies.




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here are on the heavens many hundreds of thousands of stars, and the problem of entifying the planet, if indeed it should lie among these stars, seemed a very complexatter. Of course it is the abundant presence of the stars which causes the difficulty. If thears could have been got rid of, a sweep over the heavens would at once disclose all theanets which are bright enough to be visible with the telescopic power employed. It is thertuitous resemblance of the planet to the stars which enables it to escape detection. To

scriminate the planet among stars everywhere in the sky would be almost impossible. If,owever, some method could be devised for localizing that precise region in which the planxistence might be presumed, then the search could be undertaken with some prospect of ccess.

o a certain extent the problem of localizing the region on the sky in which the planet mighe expected admitted of an immediate limitation. It is known that all the planets, or perhapught rather to say, all the great planets, confine their movements to a certain zone arounde heavens. This zone extends some way on either side of that line called the ecliptic in w

e earth pursues its journey around the sun. It was therefore to be inferred that the newanet need not be sought for outside this zone. It is obvious that this consideration at onceduces the area to be scrutinized to a small fraction of the entire heavens. But even withine zone thus defined there are many thousands of stars. It would seem a hopeless task to

etect the new planet unless some further limitation to its position could be assigned.

was accordingly suggested to Le Verrier that he should endeavour to discover in whatarticular part of the strip of the celestial sphere which we have indicated the search for thenknown planet should be instituted. The materials available to the mathematician for the

olution of this problem were to be derived solely from the discrepancies between thelculated places in which Uranus should be found, taking into account the known causes osturbance, and the actual places in which observation had shown the planet to exist. Hereas indeed an unprecedented problem, and one of extraordinary difficulty. Le Verrier,owever, faced it, and, to the astonishment of the world, succeeded in carrying it through tilliant solution. We cannot here attempt to enter into any account of the mathematicalvestigations that were necessary. All that we can do is to give a general indication of theethod which had to be adopted.

et us suppose that a planet is revolving outside Uranus, at a distance which is suggested be several distances at which the other planets are dispersed around the sun. Let us assumat this outer planet has started on its course, in a prescribed path, and that it has a certaass. It will, of course, disturb the motion of Uranus, and in consequence of that disturbanranus will follow a path the nature of which can be determined by calculation. It will,owever, generally be found that the path so ascertained does not tally with the actual pathhich observations have indicated for Uranus. This demonstrates that the assumedrcumstances of the unknown planet must be in some respects erroneous, and thetronomer commences afresh with an amended orbit. At last after many trials, Le Verrier




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certained that, by assuming a certain size, shape, and position for the unknown Planet'sbit, and a certain value for the mass of the hypothetical body, it would be possible to

ccount for the observed disturbances of Uranus. Gradually it became clear to the perceptiothis consummate mathematician, not only that the difficulties in the movements of Uranu

ould be thus explained, but that no other explanation need be sought for. It accordinglyppeared that a planet possessing the mass which he had assigned, and moving in the orbhich his calculations had indicated, must indeed exist, though no eye had ever beheld any

ch body. Here was, indeed, an astonishing result. The mathematician sitting at his desk, udying the observations which had been supplied to him of one planet, is able to discovere existence of another planet, and even to assign the very position which it must occupy,e ever the telescope is invoked for its discovery.

hus it was that the calculations of Le Verrier narrowed greatly the area to be scrutinised ine telescopic search which was presently to be instituted. It was already known, as we havst pointed out, that the planet must lie somewhere on the ecliptic. The Frenchathematician had now further indicated the spot on the ecliptic at which, according to his

lculations, the planet must actually be found. And now for an episode in this history whicll be celebrated so long as science shall endure. It is nothing less than the telescopic

onfirmation of the existence of this new planet, which had previously been indicated only bathematical calculation. Le Verrier had not himself the instruments necessary for studyinge heavens, nor did he possess the skill of the practical astronomer. He, therefore, wrote tr. Galle, of the Observatory at Berlin, requesting him to undertake a telescopic search for ew planet in the vicinity which the mathematical calculation had indicated for thehereabouts of the planet at that particular time. Le Verrier added that he thought the planught to admit of being recognised by the possession of a disc sufficiently definite to mark

stinction between it and the surrounding stars.

was the 23rd September, 1846, when the request from Le Verrier reached the Berlinbservatory, and the night was clear, so that the memorable search was made on the samvening. The investigation was facilitated by the circumstance that a diligent observer hadcently compiled elaborate star maps for certain tracts of the heavens lying in a sufficientlyde zone on both sides of the equator. These maps were as yet only partially complete, bu

appened that Hora. XXI., which included the very spot which Le Verrier's results referred tad been just issued. Dr. Galle had thus before his, eyes a chart of all the stars which were

sible in that part of the heavens at the time when the map was made. The advantage of ch an assistance to the search could hardly be over-estimated. It at once gave thetronomer another method of recognising the planet besides that afforded by its possible

ossession of a disc. For as the planet was a moving body, it would not have been in the saace relatively to the stars at the time when the map was constructed, as it occupied someears later when the search was being made. If the body should be situated in the spot whe Verrier's calculations indicated in the autumn of 1846, then it might be regarded as certaat it would not be found in that same place on a map drawn some years previously.




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he search to be undertaken consisted in a comparison made point by point between theodies shown on the map, and those stars in the sky which Dr. Galle's telescope revealed. Ie course of this comparison it presently appeared that a star-like object of the eighthagnitude, which was quite a conspicuous body in the telescope, was not represented in thap. This at once attracted the earnest attention of the astronomer, and raised his hopes tere was indeed the planet. Nor were these hopes destined to be disappointed. It could noe supposed that a star of the eighth magnitude would have been overlooked in the

eparation of a chart whereon stars of many lower degrees of brightness were set down. Oher supposition was of course conceivable. It might have been that this suspicious object

elonged to the class of variables, for there are many such stars whose brightness fluctuatend if it had happened that the map was constructed at a time when the star in question hut feeble brilliance, it might have escaped notice. It is also well known that sometimes newars suddenly develop, so that the possibility that what Dr. Galle saw should have been aariable star or should have been a totally new star had to be provided against.

ortunately a test was immediately available to decide whether the new object was indeed

ng sought for planet, or whether it was a star of one of the two classes to which I have juferred. A star remains fixed, but a planet is in motion. No doubt when a planet lies at thestance at which this new planet was believed to be situated, its apparent motion would beow that it would not be easy to detect any change in the course of a single night'sbservation. Dr. Galle, however, addressed himself with much skill to the examination of thace of the new body. Even in the course of the night he thought he detected slightovements, and he awaited with much anxiety the renewal of his observations on thebsequent evenings. His suspicions as to the movement of the body were then amply

onfirmed, and the planetary nature of the new object was thus unmistakably detected.

reat indeed was the admiration of the scientific world at this superb triumph. Here was aighty planet whose very existence was revealed by the indications afforded by refinedathematical calculation. At once the name of Le Verrier, already known to those conversath the more profound branches of astronomy, became everywhere celebrated. It soon,

owever, appeared, that the fame belonging to this great achievement had to be sharedetween Le Verrier and another astronomer, J. C. Adams, of Cambridge. In our chapter onis great English mathematician we shall describe the manner in which he was independend to the same discovery.

rectly the planetary nature of the newly-discovered body had been established, the greatbservatories naturally included this additional member of the solar system in their workingts, so that day after day its place was carefully determined. When sufficient time hadapsed the shape and position of the orbit of the body became known. Of course, it needardly be said that observations applied to the planet itself must necessarily provide a farore accurate method of determining the path which it follows, than would be possible to Lerrier, when all he had to base his calculations upon was the influence of the planetflected, so to speak, from Uranus. It may be noted that the true elements of the planet,




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hen revealed by direct observation, showed that there was a considerable discrepancyetween the track of the planet which Le Verrier had announced, and that which the planetas actually found to pursue.

he name of the newly-discovered body had next to be considered. As the older members oe system were already known by the same names as great heathen divinities, it was obviat some similar source should be invoked for a suggestion as to a name for the most rece

anet. The fact that this body was so remote in the depths of space, not unnaturallyggested the name "Neptune." Such is accordingly the accepted designation of that mightobe which revolves in the track that at present seems to trace out the frontiers of ourstem.

e Verrier attained so much fame by this discovery, that when, in 1854, Arago's place had te filled at the head of the great Paris Observatory, it was universally felt that the discovereNeptune was the suitable man to assume the office which corresponds in France to that e Astronomer Royal in England. It was true that the work of the astronomical mathematic

ad hitherto been of an abstract character. His discoveries had been made at his desk and the observatory, and he had no practical acquaintance with the use of astronomicalstruments. However, he threw himself into the technical duties of the observatory withgour and determination. He endeavoured to inspire the officers of the establishment withnthusiasm for that systematic work which is so necessary for the accomplishment of usefutronomical research. It must, however, be admitted that Le Verrier was not gifted with th

atural qualities which would make him adapted for the successful administration of such atablishment. Unfortunately disputes arose between the Director and his staff. At last thefficulties of the situation became so great that the only possible solution was to supersede

errier, and he was accordingly obliged to retire. He was succeeded in his high office bynother eminent mathematician, M. Delaunay, only less distinguished than Le Verrier himse

elieved of his official duties, Le Verrier returned to the mathematics he loved. In his non-ficial capacity he continued to work with the greatest ardour at his researches on theovements of the planets. After the death of M. Delaunay, who was accidentally drowned i873, Le Verrier was restored to the directorship of the observatory, and he continued to he office until his death.

he nature of the researches to which the life of Le Verrier was subsequently devoted are nch as admit of description in a general sketch like this, where the language, and still less mbols, of mathematics could not be suitably introduced. It may, however, be said in geneat he was particularly engaged with the study of the effects produced on the movements e planets by their mutual attractions. The importance of this work to astronomy consists, considerable extent, in the fact that by such calculations we are enabled to prepare tabley which the places of the different heavenly bodies can be predicted for our almanacs. Tois task Le Verrier devoted himself, and the amount of work he has accomplished would

erhaps have been deemed impossible had it not been actually done.




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he superb success which had attended Le Verrier's efforts to explain the cause of theerturbations of Uranus, naturally led this wonderful computer to look for a similar explanatcertain other irregularities in planetary movements. To a large extent he succeeded inowing how the movements of each of the great planets could be satisfactorily accounted

y the influence of the attractions of the other bodies of the same class. One circumstance onnection with these investigations is sufficiently noteworthy to require a few words here.

st as at the opening of his career, Le Verrier had discovered that Uranus, the outermostanet of the then known system, exhibited the influence of an unknown external body, soow it appeared to him that Mercury, the innermost body of our system, was also subjectedome disturbances, which could not be satisfactorily accounted for as consequences of anynown agents of attraction. The ellipse in which Mercury revolved was animated by a slowovement, which caused it to revolve in its plane. It appeared to Le Verrier that thissplacement was incapable of explanation by the action of any of the known bodies of ourstem. He was, therefore, induced to try whether he could not determine from thesturbances of Mercury the existence of some other planet, at present unknown, which

volved inside the orbit of the known planet. Theory seemed to indicate that the observedteration in the track of the planet could be thus accounted for. He naturally desired to obtlescopic confirmation which might verify the existence of such a body in the same way asr. Galle verified the existence of Neptune. If there were, indeed, an intramercurial planet,en it must occasionally cross between the earth and the sun, and might now and then be

xpected to be witnessed in the actual act of transit. So confident did Le Verrier feel in thexistence of such a body that an observation of a dark object in transit, by Lescarbault on6th March, 1859, was believed by the mathematician to be the object which his theorydicated. Le Verrier also thought it likely that another transit of the same object would be

en in March, 1877. Nothing of the kind was, however, witnessed, notwithstanding that ansiduous watch was kept, and the explanation of the change in Mercury's orbit must,erefore, be regarded as still to be sought for.

e Verrier naturally received every honour that could be bestowed upon a man of science. Ttter part of his life was passed during the most troubled period of modern French history. as a supporter of the Imperial Dynasty, and during the Commune he experienced muchnxiety; indeed, at one time grave fears were entertained for his personal safety.

arly in 1877 his health, which had been gradually failing for some years, began to give wae appeared to rally somewhat in the summer, but in September he sank rapidly, and died unday, the 23rd of that month.

s remains were borne to the cemetery on Mont Parnasse in a public funeral. Among hisallbearers were leading men of science, from other countries as well as France, and theemorial discourses pronounced at the grave expressed their admiration of his talents and e greatness of the services he had rendered to science.




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ADAMS.

he illustrious mathematician who, among Englishmen, at all events, was second only toewton by his discoveries in theoretical astronomy, was born on June the 5th, 1819, at thermhouse of Lidcot, seven miles from Launceston, in Cornwall. His early education was

mparted under the guidance of the Rev. John Couch Grylls, a first cousin of his mother. Heppears to have received an education of the ordinary school type in classics andathematics, but his leisure hours were largely devoted to studying what astronomical booe could find in the library of the Mechanics' Institute at Devonport. He was twenty years ohen he entered St. John's College, Cambridge. His career in the University was one of almnparalleled distinction, and it is recorded that his answering at the Wranglership examinathere he came out at the head of the list in 1843, was so high that he received more thanouble the marks awarded to the Second Wrangler.

mong the papers found after his death was the following memorandum, dated July the 3rd841: "Formed a design at the beginning of this week of investigating, as soon as possibleter taking my degree, the irregularities in the motion of Uranus, Which are as yetnaccounted for, in order to find whether they may be attributed to the action of anndiscovered planet beyond it; and, if possible, thence to determine the elements of its orbpproximately, which would lead probably to its discovery."

ter he had taken his degree, and had thus obtained a little relaxation from the lines withihich his studies had previously been necessarily confined, Adams devoted himself to theudy of the perturbations of Uranus, in accordance with the resolve which we have just seeat he formed while he was still an undergraduate. As a first attempt he made thepposition that there might be a planet exterior to Uranus, at a distance which was doubleat of Uranus from the sun. Having completed his calculation as to the effect which such a

ypothetical planet might exercise upon the movement of Uranus, he came to the conclusioat it would be quite possible to account completely for the unexplained difficulties by the

ction of an exterior planet, if only that planet were of adequate size and had its orbit propaced. It was necessary, however, to follow up the problem more precisely, and accordingn application was made through Professor Challis, the Director of the Cambridgebservatory, to the Astronomer Royal, with the object of obtaining from the observationsade at Greenwich Observatory more accurate values for the disturbances suffered by

ranus. Basing his work on the more precise materials thus available, Adams undertook hislculations anew, and at last, with his completed results, he called at Greenwich Observato

n October the 21st, 1845. He there left for the Astronomer Royal a paper which containede results at which he had arrived for the mass and the mean distance of the hypotheticalanet as well as the other elements necessary for calculating its exact position.

s we have seen in the preceding chapter, Le Verrier had been also investigating the sameoblem. The place which Le Verrier assigned to the hypothetical disturbing planet for the




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eginning of the year 1847, was within a degree of that to which Adams's computationsointed, and which he had communicated to the Astronomer Royal seven months before Leerrier's work appeared. On July the 29th, 1846, Professor Challis commenced to search foe unknown object with the Northumberland telescope belonging to the Cambridgebservatory. He confined his attention to a limited region in the heavens, extending aroundat point to which Mr. Adams' calculations pointed. The relative places of all the stars, orther star-like objects within this area, were to be carefully measured. When the same

bservations were repeated a week or two later, then the distances of the several pairs of ars from each other would be found unaltered, but any planet which happened to lie amoe objects measured would disclose its existence by the alterations in distance due to itsotion in the interval. This method of search, though no doubt it must ultimately have provccessful, was necessarily a very tedious one, but to Professor Challis, unfortunately, noher method was available. Thus it happened that, though Challis commenced his search aambridge two months earlier than Galle at Berlin, yet, as we have already explained, theossession of accurate star-maps by Dr. Galle enabled him to discover the planet on the vest night that he looked for it.

he rival claims of Adams and Le Verrier to the discovery of Neptune, or rather, we shouldy, the claims put forward by their respective champions, for neither of the illustriousvestigators themselves condescended to enter into the personal aspect of the question, not be further discussed here. The main points of the controversy have been long sincettled, and we cannot do better than quote the words of Sir John Herschel when he

ddressed the Royal Astronomical Society in 1848:--

As genius and destiny have joined the names of Le Verrier and Adams, I shall by no mean

ut them asunder; nor will they ever be pronounced apart so long as language shall celebrae triumphs Of science in her sublimest walks. On the great discovery of Neptune, which m

e said to have surpassed, by intelligible and legitimate means, the wildest pretensions of airvoyance, it Would now be quite superfluous for me to dilate. That glorious event and theps which led to it, and the various lights in which it has been placed, are already familiarvery one having the least tincture of science. I will only add that as there is not, norenceforth ever can be, the slightest rivalry on the subject between these two illustrious me they have met as brothers, and as such will, I trust, ever regard each other--we haveade, we could make, no distinction between then, on this occasion. May they both long

dorn and augment our science, and add to their own fame already so high and pure, by frchievements."

dams was elected a Fellow of St. John's College, Cambridge, in 1843; but as he did not taoly orders, his Fellowship, in accordance with the rules then existing came to an end in 18

the following year he was, however, elected to a Fellowship at Pembroke College, whichtained until the end of his life. In 1858 he was appointed Professor of Mathematics in theniversity of St. Andrews, but his residence in the north was only a brief one, for in the samear he was recalled to Cambridge as Lowndean Professor of Astronomy and Geometry, in




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ccession to Peacock. In 1861 Challis retired from the Directorship of the Cambridgebservatory, and Adams was appointed to succeed him.

he discovery of Neptune was a brilliant inauguration of the astronomical career of Adams. orked at, and wrote upon, the theory of the motions of Biela's comet; he made importantorrections to the theory of Saturn; he investigated the mass of Uranus, a subject in which as naturally interested from its importance in the theory of Neptune; he also improved the

ethods of computing the orbits of double stars. But all these must be regarded as his minbours, for next to the discovery of Neptune the fame of Adams mainly rests on hissearches upon certain movements of the moon, and upon the November meteors.

he periodic time of the moon is the interval required for one circuit of its orbit. This intervanown with accuracy at the present day, and by means of the ancient eclipses the period oe moon's revolution two thousand years ago can be also ascertained. It had been discove

y Halley that the period which the moon requires to accomplish each of its revolutions aroe earth has been steadily, though no doubt slowly, diminishing. The change thus produce

not appreciable when only small intervals of time are considered, but it becomes appreciahen we have to deal with intervals of thousands of years. The actual effect which isoduced by the lunar acceleration, for so this phenomenon is called, may be thus estimatewe suppose that the moon had, throughout the ages, revolved around the earth in precise same periodic time which it has at present, and if from this assumption we calculate bafind where the moon must have been about two thousand years ago, we obtain a positio

hich the ancient eclipses show to be different from that in which the moon was actuallytuated. The interval between the position in which the moon would have been found twoousand years ago if there had been no acceleration, and the position in which the moon w

ctually placed, amounts to about a degree, that is to say, to an arc on the heavens which wice the moon's apparent diameter.

no other bodies save the earth and the moon were present in the universe, it seems certat the motion of the moon would never have exhibited this acceleration. In such a simplese as that which I have supposed the orbit of the moon would have remained for ever

bsolutely unchanged. It is, however, well known that the presence of the sun exerts asturbing influence upon the movements of the moon. In each revolution our satellite is

ontinually drawn aside by the action of the sun from the place which it would otherwise ha

ccupied. These irregularities are known as the perturbations of the lunar orbit, they haveng been studied, and the majority of them have been satisfactorily accounted for. It seemowever, to those who first investigated the question that the phenomenon of the lunarcceleration could not be explained as a consequence of solar perturbation, and, as no othegent competent to produce such effects was recognised by astronomers, the lunarcceleration presented an unsolved enigma.

the end of the last century the illustrious French mathematician Laplace undertook a newvestigation of the famous problem, and was rewarded with a success which for a long tim




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ppeared to be quite complete. Let us suppose that the moon lies directly between the eartnd the sun, then both earth and moon are pulled towards the sun by the solar attraction; owever, the moon is the nearer of the two bodies to the attracting centre it is pulled theore energetically, and consequently there is an increase in the distance between the earth

nd the moon. Similarly when the moon happens to lie on the other side of the earth, so the earth is interposed directly between the moon and the sun, the solar attraction exerted

pon the earth is more powerful than the same influence upon the moon. Consequently in

se, also, the distance of the moon from the earth is increased by the solar disturbance.hese instances will illustrate the general truth, that, as one of the consequences of thesturbing influence exerted by the sun upon the earth-moon system, there is an increase ine dimensions of the average orbit which the moon describes around the earth. As the timquired by the moon to accomplish a journey round the earth depends upon its distance fre earth, it follows that among the influences of the sun upon the moon there must be an

nlargement of the periodic time, from what it would have been had there been no solarsturbing action.

his was known long before the time of Laplace, but it did not directly convey any explanatthe lunar acceleration. It no doubt amounted to the assertion that the moon's periodic ti

as slightly augmented by the disturbance, but it did not give any grounds for suspecting tere was a continuous change in progress. It was, however, apparent that the periodic timas connected with the solar disturbance, so that, if there were any alteration in the amouthe sun's disturbing effect, there must be a corresponding alteration in the moon's period

me. Laplace, therefore, perceived that, if he could discover any continuous change in thebility of the sun for disturbing the moon, he would then have accounted for a continuoushange in the moon's periodic time, and that thus an explanation of the long-vexed questio

the lunar acceleration might be forthcoming.

he capability of the sun for disturbing the earth-moon system is obviously connected with stance of the earth from the sun. If the earth moved in an orbit which underwent no chanhatever, then the efficiency of the sun as a disturbing agent would not undergo any changthe kind which was sought for. But if there were any alteration in the shape or size of the

arth's orbit, then that might involve such changes in the distance between the earth and tn as would possibly afford the desired agent for producing the observed lunar effect. It is

nown that the earth revolves in an orbit which, though nearly circular, is strictly an ellipse.

e earth were the only planet revolving around the sun then that ellipse would remainnaltered from age to age. The earth is, however, only one of a large number of planets whrculate around the great luminary, and are guided and controlled by his supreme attractinower. These planets mutually attract each other, and in consequence of their mutualtractions the orbits of the planets are disturbed from the simple elliptic form which theyould otherwise possess. The movement of the earth, for instance, is not, strictly speakingerformed in an elliptical orbit. We may, however, regard it as revolving in an ellipse provide admit that the ellipse is itself in slow motion.




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is a remarkable characteristic of the disturbing effects of the planets that the ellipse in whe earth is at any moment moving always retains the same length; that is to say, its longeameter is invariable. In all other respects the ellipse is continually changing. It alters itsosition, it changes its plane, and, most important of all, it changes its eccentricity. Thus, frge to age the shape of the track which the earth describes may at one time be growing mearly a circle, or at another time may be departing more widely from a circle. Theseterations are very small in amount, and they take place with extreme slowness, but they a

incessant progress, and their amount admits of being accurately calculated. At the preseme, and for thousands of years past, as well as for thousands of years to come, theccentricity of the earth's orbit is diminishing, and consequently the orbit described by thearth each year is becoming more nearly circular. We must, however, remember that undercumstances the length of the longest axis of the ellipse is unaltered, and consequently thze of the track which the earth describes around the sun is gradually increasing. In otherords, it may be said that during the present ages the average distance between the earthnd the sun is waxing greater in consequence of the perturbations which the earthxperiences from the attraction of the other planets. We have, however, already seen that

ficiency of the solar attraction for disturbing the moon's movement depends on the distanetween the earth and the sun. As therefore the average distance between the earth and tn is increasing, at all events during the thousands of years over which our observations

xtend, it follows that the ability of the sun for disturbing the moon must be graduallyminishing.

has been pointed out that, in consequence of the solar disturbance, the orbit of the moonust be some what enlarged. As it now appears that the solar disturbance is on the wholeeclining, it follows that the orbit of the moon, which has to be adjusted relatively to the

verage value of the solar disturbance, must also be gradually declining. In other words, thoon must be approaching nearer to the earth in consequence of the alterations in the

ccentricity of the earth's orbit produced by the attraction of the other planets. It is true thae change in the moon's position thus arising is an extremely small one, and the consequefect in accelerating the moon's motion is but very slight. It is in fact almost imperceptible,

xcept when great periods of time are involved. Laplace undertook a calculation on thisbject. He knew what the efficiency of the planets in altering the dimensions of the earth'sbit amounted to; from this he was able to determine the changes that would be propagatto the motion of the moon. Thus he ascertained, or at all events thought he had ascertain

at the acceleration of the moon's motion, as it had been inferred from the observations oe ancient eclipses which have been handed down to us, could be completely accounted fo a consequence of planetary perturbation. This was regarded as a great scientific triumphur belief in the universality of the law of gravitation would, in fact, have been seriouslyhallenged unless some explanation of the lunar acceleration had been forthcoming. For abty years no one questioned the truth of Laplace's investigation. When a mathematician of

minence had rendered an explanation of the remarkable facts of observation which seemeo complete, it is not surprising that there should have been but little temptation to doubt in undertaking a new calculation of the same question, Professor Adams found that Laplac




Great Astronomers

ad not pursued this approximation sufficiently far, and that consequently there was aonsiderable error in the result of his analysis. Adams, it must be observed, did not impugne value of the lunar acceleration which Halley had deduced from the observations, but wh

e did show was, that the calculation by which Laplace thought he had provided anxplanation of this acceleration was erroneous. Adams, in fact, proved that the planetaryfluence which Laplace had detected only possessed about half the efficiency which the grench mathematician had attributed to it. There were not wanting illustrious mathematicia

ho came forward to defend the calculations of Laplace. They computed the question anewnd arrived at results practically coincident with those he had given. On the other hand certstinguished mathematicians at home and abroad verified the results of Adams. The issueas merely a mathematical one. It had only one correct solution. Gradually it appeared thaose who opposed Adams presented a number of different solutions, all of them discordanth his, and, usually, discordant with each other. Adams showed distinctly where each of ese investigators had fallen into error, and at last it became universally admitted that theambridge Professor had corrected Laplace in a very fundamental point of astronomicaleory.

hough it was desirable to have learned the truth, yet the breach between observation andlculation which Laplace was believed to have closed thus became reopened. Laplace'svestigation, had it been correct, would have exactly explained the observed facts. It was,owever, now shown that his solution was not correct, and that the lunar acceleration, wherictly calculated as a consequence of solar perturbations, only produced about half the effhich was wanted to explain the ancient eclipses completely. It now seems certain that theno means of accounting for the lunar acceleration as a direct consequence of the laws of avitation, if we suppose, as we have been in the habit of supposing, that the members of

e solar system concerned may be regarded as rigid particles. It has, however, beenggested that another explanation of a very interesting kind may be forthcoming, and thisust endeavour to set forth.

will be remembered that we have to explain why the period of revolution of the moon isow shorter than it used to be. If we imagine the length of the period to be expressed inrms of days and fractions of a day, that is to say, in terms of the rotations of the earthound its axis, then the difficulty encountered is, that the moon now requires for each of itvolutions around the earth rather a smaller number of rotations of the earth around its ax

an used formerly to be the case. Of course this may be explained by the fact that the monow moving more swiftly than of yore, but it is obvious that an explanation of quite afferent kind might be conceivable. The moon may be moving just at the same pace as evut the length of the day may be increasing. If the length of the day is increasing, then, of ourse, a smaller number of days will be required for the moon to perform each revolutionven though the moon's period was itself really unchanged. It would, therefore, seem as if henomenon known as the lunar acceleration is the result of the two causes. The first of ththat discovered by Laplace, though its value was overestimated by him, in which the

erturbations of the earth by the planets indirectly affect the motion of the moon. The




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maining part of the acceleration of our satellite is apparent rather than real, it is not that oon is moving more quickly, but that our time-piece, the earth, is revolving more slowly, athus actually losing time. It is interesting to note that we can detect a physical explanatior the apparent checking of the earth's motion which is thus manifested. The tides which e

nd flow on the earth exert a brake-like action on the revolving globe, and there can be nooubt that they are gradually reducing its speed, and thus lengthening the day. It hasccordingly been suggested that it is this action of the tides which produces the

pplementary effect necessary to complete the physical explanation of the lunar acceleratiough it would perhaps be a little premature to assert that this has been fully demonstrate

he third of Professor Adams' most notable achievements was connected with the greatower of November meteors which astonished the world in 1866. This splendid display

oncentrated the attention of astronomers on the theory of the movements of the little objey which the display was produced. For the definite discovery of the track in which theseodies revolve, we are indebted to the labours of Professor Adams, who, by a brilliant pieceathematical work, completed the edifice whose foundations had been laid by Professor

ewton, of Yale, and other astronomers.

eteors revolve around the sun in a vast swarm, every individual member of which pursuesbit in accordance with the well-known laws of Kepler. In order to understand the movemethese objects, to account satisfactorily for their periodic recurrence, and to predict the timtheir appearance, it became necessary to learn the size and the shape of the track whiche swarm followed, as well as the position which it occupied. Certain features of the track

ould no doubt be readily assigned. The fact that the shower recurs on one particular day oe year, viz., November 13th, defines one point through which the orbit must pass. The

osition on the heavens of the radiant point from which the meteors appear to diverge, givnother element in the track. The sun must of course be situated at the focus, so that onlyne further piece of information, namely, the periodic time, will be necessary to complete onowledge of the movements of the system. Professor H. Newton, of Yale, had shown that hoice of possible orbits for the meteoric swarm is limited to five. There is, first, the greatipse in which we now know the meteors revolve once every thirty three and one quarter

ears. There is next an orbit of a nearly circular kind in which the periodic time would be atle more than a year. There is a similar track in which the periodic time would be a few daort of a year, while two other smaller orbits would also be conceivable. Professor Newton

ad pointed out a test by which it would be possible to select the true orbit, which we knowust be one or other of these five. The mathematical difficulties which attended the

pplication of this test were no doubt great, but they did not baffle Professor Adams.

here is a continuous advance in the date of this meteoric shower. The meteors now cross ack at the point occupied by the earth on November 13th, but this point is gradually alterhe only influence known to us which could account for the continuous change in the planee meteor's orbit arises from the attraction of the various planets. The problem to be solveay therefore be attacked in this manner. A specified amount of change in the plane of the




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bit of the meteors is known to arise, and the changes which ought to result from thetraction of the planets can be computed for each of the five possible orbits, in one of whiis certain that the meteors must revolve. Professor Adams undertook the work. Its difficuincipally arises from the high eccentricity of the largest of the orbits, which renders the mdinary methods of calculation inapplicable. After some months of arduous labour the woras completed, and in April, 1867, Adams announced his solution of the problem. He showat if the meteors revolved in the largest of the five orbits, with the periodic time of thirty

ree and one quarter years, the perturbations of Jupiter would account for a change to thextent of twenty minutes of arc in the point in which the orbit crosses the earth's track. Thetraction of Saturn would augment this by seven minutes, and Uranus would add one minuore, while the influence of the Earth and of the other planets would be inappreciable. The

ccumulated effect is thus twenty-eight minutes, which is practically coincident with thebserved value as determined by Professor Newton from an examination of all the showershich there is any historical record. Having thus showed that the great orbit was a possibleath for the meteors, Adams next proved that no one of the other four orbits would besturbed in the same manner. Indeed, it appeared that not half the observed amount of

hange could arise in any orbit except in that one with the long period. Thus was brought tompletion the interesting research which demonstrated the true relation of the meteor swa

the solar system.

esides those memorable scientific labours with which his attention was so largely engagedofessor Adams found time for much other study. He occasionally allowed himself to

ndertake as a relaxation some pieces of numerical calculation, so tremendously long that w

R. S. Ball, Great Astronomers

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